Multiplicity was working fine for months, now it's not...

Things rarely "just break" on PCs.  What else have you installed recently?  (Or uninstalled . . or looked at funny) 

How long has it not been working?

You might want to consider restoring each computer back to before Multiplicity stopped working.

Stopped this morning...

Sat down, was checking my email, then BAM!, wouldn't jump across to the other computer.

I haven't installed any new software in quite a while, so I don't think that's it. I think Windows updated last night, but like I said, it was working for a few minutes this morning before it decided to stop. Has me perplexed. I can certainly see if I can restore, and see if that works, but I'd REALLY like to get to the bottom of this, and know what to look for if there's a next time.

Check to see what windows updated, then try to determine if that may be the cause of the problem before you try a restore.

This my sound silly or you all ready did this but make sure nothing came 'unpluged' or 'disconnected'.

Both systems have network and Internet access.

The only update that would qualify in terms of timing is a single update for Windows Defender definitions. Multiplicity is on the list of allowed programs on both systems. In fact, every single file in the Multiplicity folder is on the allowed list on both systems. The port is also allowed on both systems.

And, yes, everything is plugged in and connected.

I've grown so accustomed to Multiplicity, that being without the functionality for the past 2 days has had an affect on my ability to work efficiently. There has to be an obvious cause.

Can you telnet on the default port back and forth?