Thx. I already rebooted many times since the issue appeared, but this didn't help. I uninstalled MultiMiner and reinstalled it again, but even this didn't help.
It seems that MultiMiner isn't communicating any state. Even using the MobileMiner WebApp I can't see my second computer (called 'Pinta').
I tried switching off the firewall. No positive result.
In that case you have another application that is listening on the exact same port that MultiMiner uses for Remoting. Disable MultiMiner Remoting in the Perks dialog to fix the issue.
If MultiMiner Remoting is something you were running intentionally (some folks just enable things) let me know and I can look into making the ports configurable.
I've personally only ever seen that error when MultiMiner crashes and I reopen it (with Remoting enabled).