The problem seems to be solved now. Thanks to the developers.
Trials after updating to NC 12.0.2 and Collabora Office 2.1.3 with active Collabora Online 1.12.34 App showed no issues anymore. The users were able to save the documents opened from a group folder by the Collabora Office GUI again.
This was a hassle for some time:
On NC 12.0.0 as admin I created a group folder with Group Folder App, created a file for Collabora Office 2.1.2 there with a user from that group, and could not edit erratically and could not save it regularly
Maybe other users are aware of the Group Folder + Collabora Online = Not working #70 issue too?
A workaround would be to use sharing but cannot be administered much. Unfortunately, this issue makes group folders somewhat useless to our team.