I just sent this to support as well, posting here for anyone else with the issue.
It appears that I am no longer able to add any new tables to any apps that are owned by other accounts, but that I’m an editor on. I get this screen:
I’ve tested this across multiple co-authored apps, same thing. Have tested it inside a new private browser window, same thing. Have tested it using another appsheet account login, same thing.
This does not appear to affect app owners being able to add new tables.