8.5.7 および 8.5.9 から 9.1.1 へのアップデートでエラー

concrete ディレクトリを 9.1.1 へ置き換えて以下を実行したところエラーを生じました。
concrete/bin/concrete5 c5:update

Less_Exception_Parser
File /インストールディレクトリ/concrete/css/build/vendor/font-awesome/variables.less not found.

上記ファイルは 8.5.7 や 8.5.9 にはありますが 9.1.1 には含まれていないようです。

キャッシュはすべて無効でデバッグモードは有効になっています。
エックスサーバーの WAF をすべて無効にするなど試してみましたが同じ結果でした。
どのような原因が考えられますでしょうか。

** Executing migration: 20191029175713
Refreshing schema for Concrete\Core\Entity\Attribute\Key\ExpressKey…
Refreshing schema for Concrete\Core\Entity\Board\Board…
Refreshing schema for Concrete\Core\Entity\Board\BoardPermissionAssignment…
Refreshing schema for Concrete\Core\Entity\Board\Instance…
Refreshing schema for Concrete\Core\Entity\Board\InstanceItem…
Refreshing schema for Concrete\Core\Entity\Board\InstanceItemBatch…
Refreshing schema for Concrete\Core\Entity\Board\Item…
Refreshing schema for Concrete\Core\Entity\Board\ItemCategory…
Refreshing schema for Concrete\Core\Entity\Board\ItemTag…
Refreshing schema for Concrete\Core\Entity\Board\InstanceSlot…
Refreshing schema for Concrete\Core\Entity\Board\InstanceSlotRule…
Refreshing schema for Concrete\Core\Entity\Board\Template…
Refreshing schema for Concrete\Core\Entity\Board\SlotTemplate…
Refreshing schema for Concrete\Core\Entity\Board\DataSource\DataSource…
Refreshing schema for Concrete\Core\Entity\Board\DataSource\ConfiguredDataSource…
Refreshing schema for Concrete\Core\Entity\Board\DataSource\Configuration\Configuration…
Refreshing schema for Concrete\Core\Entity\Board\DataSource\Configuration\PageConfiguration…
Refreshing schema for Concrete\Core\Entity\Board\DataSource\Configuration\CalendarEventConfiguration…
Refreshing schema for Concrete\Core\Entity\Page\Container…
Refreshing schema for Concrete\Core\Entity\Page\Container\Instance…
Refreshing schema for Concrete\Core\Entity\Page\Container\InstanceArea…
Refreshing schema for Concrete\Core\Entity\Summary\Template…
Refreshing schema for Concrete\Core\Entity\Summary\Category…
Refreshing schema for Concrete\Core\Entity\Summary\Field…
Refreshing schema for Concrete\Core\Entity\Summary\TemplateField…
Refreshing schema for Concrete\Core\Entity\Page\Summary\CustomPageTemplateCollection…
Refreshing schema for Concrete\Core\Entity\Page\Summary\PageTemplate…
Refreshing schema for Concrete\Core\Entity\Calendar\CalendarEvent…
Refreshing schema for Concrete\Core\Entity\Calendar\Summary\CalendarEventTemplate…
Installing boards and containers upgrade XML…
Installing core boards and summaries XML…
** Executing migration: 20200116115000
** Executing migration: 20200523051311
Updating tables found in doctrine xml…
Refreshing schema for Concrete\Core\Entity\Express\Entity…
Refreshing schema for Concrete\Core\Entity\Express\Entry…
** Executing migration: 20200604000000
** Executing migration: 20200610162600
Refreshing schema for Concrete\Core\Entity\File\File…
Refreshing schema for Concrete\Core\Entity\File\DownloadStatistics…
** Executing migration: 20200611161800
Refreshing schema for Concrete\Core\Entity\Site\Site…
Creating single page at /dashboard/files/details…
** Executing migration: 20200625191946
** Executing migration: 20200626142348
Refreshing schema for Concrete\Core\Entity\Board\Designer\CustomElement…
Refreshing schema for Concrete\Core\Entity\Board\Designer\ItemSelectorCustomElement…
Refreshing schema for Concrete\Core\Entity\Board\Designer\ItemSelectorCustomElementItem…
Creating single page at /dashboard/boards/designer…
Creating single page at /dashboard/boards/designer/choose_items…
Creating single page at /dashboard/boards/designer/customize_slot…
Creating single page at /dashboard/boards/scheduler…
Creating single page at /dashboard/boards/instances/rules…
** Executing migration: 20200810000000
Refreshing schema for Concrete\Core\Entity\Search\SavedLogSearch…
Installing log permissions upgrade XML…
** Executing migration: 20200814143401
Refreshing block type calendar_event
** Executing migration: 20200903201537
Refreshing schema for Concrete\Core\Entity\Search\SavedExpressSearch…
** Executing migration: 20200923143317
Refreshing schema for Concrete\Core\Entity\Design\DesignTag…
Refreshing schema for Concrete\Core\Entity\Board\SlotTemplate…
Refreshing schema for Concrete\Core\Entity\Summary\Template…
Refreshing schema for Concrete\Core\Entity\Board\DataSource\Configuration\CalendarEventConfiguration…
** Executing migration: 20200928000000
** Executing migration: 20200928000001
Creating single page at /dashboard/system/mail/logging…
** Executing migration: 20200928000003
** Executing migration: 20201015174802
Refreshing schema for Concrete\Core\Entity\Express\Entry…
** Executing migration: 20201018000000
Refreshing schema for Concrete\Core\Entity\File\ExternalFileProvider\ExternalFileProvider…
Refreshing schema for Concrete\Core\Entity\File\ExternalFileProvider\Type\Type…
Creating single page at /dashboard/system/files/external_file_provider…
** Executing migration: 20201022000000
Refreshing schema for Concrete\Core\Entity\File\Folder\FavoriteFolder…
** Executing migration: 20201028143317
Refreshing schema for Concrete\Core\Entity\User\User…
** Executing migration: 20201028201040
Refreshing block type express_entry_list
** Executing migration: 20201031143314
Refreshing schema for Concrete\Core\Entity\File\File…
** Executing migration: 20201110150900
Refreshing block type gallery
** Executing migration: 20201112172200
Refreshing schema for Concrete\Core\Entity\File\Image\Editor…
Creating single page at /dashboard/system/files/image_editor…
** Executing migration: 20201119123200
Creating single page at /dashboard/users/groups/bulk_user_assignment…
** Executing migration: 20201130130221
** Executing migration: 20201205023211
Refreshing block type date_navigation
Refreshing block type event_list
Refreshing block type express_entry_list
Refreshing block type feature
Refreshing block type page_list
Refreshing block type rss_displayer
Refreshing block type topic_list
Refreshing block type google_map
** Executing migration: 20201229143500
** Executing migration: 20201231153200
Refreshing schema for Concrete\Core\Entity\Attribute\Value\Value\DurationValue…
Refreshing schema for Concrete\Core\Entity\Attribute\Key\Settings\DurationSettings…
** Executing migration: 20210128224608
Refreshing block type document_library
** Executing migration: 20210205193115
Refreshing block type express_entry_list
** Executing migration: 20210216184000
Refreshing schema for Concrete\Core\Entity\Search\SavedGroupSearch…
Refreshing schema for Concrete\Core\Entity\Notification\GroupRoleChangeNotification…
Refreshing schema for Concrete\Core\Entity\Notification\GroupSignupNotification…
Refreshing schema for Concrete\Core\Entity\Notification\GroupSignupRequestNotification…
Refreshing schema for Concrete\Core\Entity\User\GroupRoleChange…
Refreshing schema for Concrete\Core\Entity\User\GroupSignup…
Refreshing schema for Concrete\Core\Entity\User\GroupSignupRequest…
Refreshing schema for Concrete\Core\Entity\Notification\GroupSignupRequestDeclineNotification…
Refreshing schema for Concrete\Core\Entity\User\GroupSignupRequestDecline…
Refreshing schema for Concrete\Core\Entity\Notification\GroupSignupRequestAcceptNotification…
Refreshing schema for Concrete\Core\Entity\User\GroupSignupRequestAccept…
Refreshing schema for Concrete\Core\Entity\Notification\GroupCreateNotification…
Refreshing schema for Concrete\Core\Entity\User\GroupCreate…
Creating single page at /dashboard/users/group_types…
Transforming old permission key search_users_in_group to new permission key search_group_folder for tree node ID 1
Transforming old permission key edit_group to new permission key edit_group_folder for tree node ID 1
Transforming old permission key assign_group to new permission key assign_groups for tree node ID 1
Transforming old permission key add_sub_group to new permission key add_group for tree node ID 1
Transforming old permission key edit_group_permissions to new permission key edit_group_folder_permissions for tree node ID 1
Copying permissions based on permission key edit_group_folder_permissions to permission key delete_group_folder for tree node ID 1
Copying permissions based on permission key edit_group_folder_permissions to permission key add_group_folder for tree node ID 1
** Executing migration: 20210331213100
** Executing migration: 20210428212600
** Executing migration: 20210528170900
Refreshing schema for Concrete\Core\Entity\User\User…
** Executing migration: 20210616131513
Refreshing block type testimonial
** Executing migration: 20210712110100
Refreshing block type image
** Executing migration: 20210718190101
Refreshing schema for Concrete\Core\Entity\Site\Site…
** Executing migration: 20210722225853
Refreshing schema for Concrete\Core\Entity\Page\Theme\CustomSkin…
** Executing migration: 20210725000000
Creating single page at /dashboard/system/automation…
Creating single page at /dashboard/system/notification…
Creating single page at /dashboard/system/automation/tasks…
Creating single page at /dashboard/system/automation/activity…
Creating single page at /dashboard/system/automation/schedule…
Creating single page at /dashboard/system/automation/settings…
Creating single page at /dashboard/system/notification/events…
Creating single page at /dashboard/system/notification/alerts…
Refreshing schema for Concrete\Core\Entity\Automation\Task…
Refreshing schema for Concrete\Core\Entity\Automation\TaskSet…
Refreshing schema for Concrete\Core\Entity\Automation\TaskSetTask…
Refreshing schema for Concrete\Core\Entity\Command\Batch…
Refreshing schema for Concrete\Core\Entity\Command\Process…
Refreshing schema for Concrete\Core\Entity\Command\TaskProcess…
Installing automated tasks upgrade XML…
** Executing migration: 20210729191135
** Executing migration: 20210813173441
** Executing migration: 20210815000000
Refreshing schema for Concrete\Core\Entity\Page\Feed…
Refreshing block type page_list
Creating global single page at /!themes…
** Executing migration: 20210910234801
** Executing migration: 20210926145823
Updating database tables found in doctrine xml…
** Executing migration: 20211001145301
** Executing migration: 20211020151701
Creating single page at /dashboard/system/permissions/denylist…
Creating single page at /dashboard/system/permissions/denylist/configure…
Creating single page at /dashboard/system/permissions/denylist/range…
** Executing migration: 20211023155414
Upgrading any existing theme customizer values…
Backing up style customizer values…
Updating database tables found in doctrine xml…
Reading style customizer lists from backup location…
No legacy values requiring upgrade were found.
** Executing migration: 20211028000000
** Executing migration: 20211102171600
Checking rich text editor settings for site: default
Editor config found.
Found concrete5link plugin in editor config. Updating to concretelink
Found concrete5styles plugin in editor config. Updating to concretestyles
Updating config values for new plugins…
** Executing migration: 20211102171700
Refreshing schema for Concrete\Core\Entity\Validation\BannedWord…
** Executing migration: 20211103142609
Updating tables found in doctrine xml…
** Executing migration: 20211104161958
Refreshing schema for Concrete\Core\Entity\Command\ScheduledTask…
** Executing migration: 20211208145933
** Executing migration: 20220114215506
Refreshing block type accordion
** Executing migration: 20220210120000
Refreshing block type feature_link
** Executing migration: 20220301185614
** Executing migration: 20220304163335
Creating single page at /dashboard/system/environment/security…
** Executing migration: 20220307105409
Refreshing schema for Concrete\Core\Entity\StyleCustomizer\Inline\StyleSet…
** Executing migration: 20220321122800
Refreshing block type hero_image
Refreshing schema for Concrete\Core\Entity\Attribute\Key\ExpressKey…
Refreshing schema for Concrete\Core\Entity\Express\Entity…
** Executing migration: 20220331222557
Refreshing block type express_form
** Executing migration: 20220408000000
** Executing migration: 20220503000000
Refreshing block type video
** Executing migration: 20220516191423
Attribute key exclude_subpages_from_nav not found. Creating…

Concrete CMS バージョン
8.5.7
8.5.9

PHP バージョン
7.4.x

サーバー
エックスサーバー

そのファイルは、バージョン9には含まれていませんし、使われてもいません。
おそらく、何かのアドオンがconcreteフォルダ内のそのファイルを勝手に使っていたんだと思います。
そのアドオンをアンインストールするか、もし最新版が出ていたらバージョンアップすべきでしょう。
Manual Nav が怪しいと思います。

ご明察の通り Manual Nav 2.3.4 が使用されており、アンインストールすると本件エラーは解消しました。
[管理画面]-[Extend concrete5]-[マニュアルナビ][詳細]-[パッケージをアンインストール]

バージョン 9 対応の Manual Nav 3.0.0 へのアップデートは バージョン 8 サイトでは以下のように表示されるため
[このパッケージはconcrete5のバージョン 9.0 以上が必要です]

バージョン 9 へアップデート後に対応という手順なのかと考えておりましたが、一旦アンインストールする方法で進めることにいたします。
早々にご回答くださいましてありがとうございました。