finally everything seems to work again!
@BernhardPosselt: thanks I must have been blind yesterday… was way too late…
I found the entry “apps.json” in the oc_filecache table and removed it as well as two other entries with the values “appstore” and “categories” in their path.
One more info for the app-devs of the app “Video calls” (https://github.com/nextcloud/spreed), when I tried to reactivate it, an exception came up saying: that the tables (oc_spreedme_messages, oc_spreedme_rooms, oc_spreedme_room_participants) the app is using, already exists.
Well normally they shouldn’t already exist, but I think if they would check in this case if the table exists they can jump to the next config-step. Because if they don’t need to be rebuild, no valuable data in theses three tables is getting lost. I had nothing special in there, but might be different for bigger instances…