All uploads to groupfolders results in error message

I am setting up a system of group folders. But the problem I have, is that registered users cannot upload files in the group folders. Whenever they try, the progress bar goes to 100% upload and then an error message appears, which says: “Path needs to be relative to the system wide data folder and point to a user specific file”. This message disappears when you just click somewhere. And no file appears in the folder or has been uploaded.

I’m even having problems decoding what that message means. So I don’t know where to start looking for the problem. I could use some help with this.

To give a more complete picture, I will describe the setup I got going on. I want to control exactly which of my users that has access to what folders. So, every groupfolder I have created, is connected to one(1) usergroup. So by selecting which groups a certain user can access, I can access which groupfolder every individual user can access.
I also don’t want the users to be able to share files or folders. As Admin I will take care of whatever sharing needs to be done. To prevent users from sharing, I have set almost all groups to “Exclude groups from sharing” in Settings > Sharing. This seems to be relevant in however files can be uploaded to a groupfolder.

A funny variation on this, is that if I upload a file to a groupfolder, I get the above error message I described above. But if I try to upload the exact same file once again, then I do not get the errormessage decribed above. Instead I get a dialogue box from Nextcloud saying something along the lines of ‘A file with the same name already exists in this folder. Do you wish to keep the existing one, or overwrite with the newly uploaded file?’.
If I chose to overwrite, then the file does appear in the groupfolder - the very same files that prompted the error message above. I’m getting a funny feeling that I might have stumbled on a bug.

IF anyone can help me understand the error message above and what I may be able to do, to unlock the situation and/or understand what causes the error message above.

Thanks in advance