If a user that belongs to a group is assigned to a case role as a group instead of as a user by mistake, the current case can't be continued because of error which is thrown in UI during user task work flow - this is an expected behavior.
The problems occur for any new started cases. Even when the assignment of users/groups is performed properly, none of the cases can be completed because the user/group scheme introduced with the case with the rogue assignment is persisted.

The problem stays even when the problematic case is closed or when the whole case project is re-deployed to Kie Server.
It seems that only restarting the application server and re-deploying Showcase App erase the persisted users/groups scheme.