There seems to be a 5GB quota on Group Folders.
See the following screenshot:
How can I increase the quota of 5GB for a group folder?
The users of these folders all have “unlimited” quota, which in our case is 100GB.
Thank you for a helpful hint, how to change this setting.
As Administrator choose the menu “Settings” and then in the menu on the left almost all at the bottom you will see “Group Folders”. Once you click on that menu you will see all the group folders and you can adjust the capacity of a group folder.