Recently I decided to maintain an old Nextcloud installation of mine. I moved it to a new server and upgraded it from version 20 to 24 (step by step). I also enabled the groupfolders feature and moved some folders and files from a user to the new groupfolders.
At first everything seemed to work. After a while I got some complaints about files that could not be opened with the web interface. Upon further investigation I saw that the file was encrypted (this is quite easy to see as its content starts with “HBEGIN:oc_encryption_”). There were more and more reports about such files.