I tried updating a site running 8.4.4 to 8.5.15 in preparation of updating to V9 afterwards. I used the manual update method, as 8.4.4 didn’t have the ability to update from the UI.
I’ve done Concrete updates many times, with no issue.
This time I am getting an error saying:
An unexpected error occurred.
Class Concrete\Core\Permission\Access\Entity\SiteGroupEntity does not exist
I can’t get past that to login to see the error reporting.
Are you able to post your environment info here? Wondering if some migrations might not have run during the update or something and it might give us some clues.