Here is the answer:
https://community.skype.com/t5/Linux/Skype-group-chat-not-working-anymore/td-p/3987288
In short:
open a chat window and type the command
- /dumpmsnp – show details about your connection
- /showplaces – show where you are currently connected
- /msnp24 To logout
- Restart Skype
- It works again