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