Friends management is broken

Hello
I'm using Stickies for a long time for my company users, always worked like a charm.
Suddenly this morning all the friends management and the relative features stopped working. Summarizing:

• All friends have disappeared / deleted, on all Stickies installations on all PCs, and on the server as well
All clients have the auto sync enabled with the server, so I guess the list on the server was deleted first and then the empty list was synced with all clients
• Now I must re-create the friends list, but I can't add them to groups anymore. I can create groups, but can't add friends to the groups; drag-and-drop no longer works (nothing happens). The include/exclude in groups buttons don't work neither.

This is what I tried to do, without success:
• Reinstalled Stickies (ver 10_1a) but nothing changes
• Checked Windows Firewall permissions, but they're OK (all Stikies entries allowed, both for domain and private context)
• Restarted Stickies server (v2.1c)
• Restarted the PC(s)
• Antivirus (Sophos) didn't detect any threat on any machine

I'm on Windows 10 Pro - 21H2 - build 19044.1566

Any idea on what's happening here and how to solve the problem?

From the helpfile to add friend to a group :

" If you drop onto a group, with the shift key held down the dragged item will go into that group."

Great!
I was deceived by the fact that all "friends" disappeared and I thought that anything was not working anymore, so I didn't even look at the help file.
Thank you