Seems like some versions of Avast / AVG / BitDefender likes to block WebSockets, while some versions of them allow usage of port 843. Unfortunately, XIM uses port 8081, which might cause problems. 

So if you have any of the software listed above installed and have problems signing into XIM (get the “Server down” message), you might want to temporarily disable them, or if possible, add port 8081 to their whitelist in order to use XIM.

I apologize for this, but it’s beyond my control.