Failure on stacks after upgrade from 5.7 to 9.3.4

Developing with Concrete
1

Hi, I just made a huge upgrade from a 5.7 site to version 9.3.4. I went through several intermediate steps and overall it went nicely.
However I have a major problem with stacks and global areas. First of all, they’re all gone from /dashboard/blocks/stacks and I have no idea why. I can still see them from the sitemap view if I enable system pages but this will be a problem for contributors.
But most of all, it’s impossible to edit them, since I get a call to member function from null exception. It seems to be a problem with localization - the website has too locals, en_US and fr_FR and the stacks are localized : $stackOrFolder->getNeutralStack() returns null and this is not handled by ConcreteCMS code.
Any idea on how to solve this ? I have no problem editing the database, I would rather avoid editing the core, but in last ressort I can.

Call stack :
Error
…/controllers/single_page/dashboard/blocks/stacks.php:76

Concrete\Controller\SinglePage\Dashboard\Blocks\Stacks
…/src/Controller/AbstractController.php:318

Concrete\Core\Controller\AbstractController runAction
…/src/Http/ResponseFactory.php:193

Concrete\Core\Http\ResponseFactory controller
…/src/Http/ResponseFactory.php:372

Concrete\Core\Http\ResponseFactory collection
…/src/Http/DefaultDispatcher.php:132

2

Missing class issues after an update can sometimes be resolved by clearing the cache and refreshing database entities.

As a general rule, I always disable and clear all caches and set database entities into development mode during an update.

3

Yes, I did that, caches are clear and entities are in dev mode. The problem seems to be that during the upgrade there was some code change or something in how localized stacks are handled and the database does not reflect that. If I set stMultilingualSection to 0, I can see my stacks again.