Livechat hanging in list of incomming chats - room not found - not deletable

Hi. We have one very old support chat hanging: in the desktop client it appears in “incomming chats”. When clicking on it it says “room not found”. Also in Livechat admin / Current chats it is not in the list

=> how can I delete this weird hanging thing? Is this something I need to delete in MongoDB directly? how would I do that?

Thanks for any advise
Regards
Petr