I suspect the firewall on the primary is blocking incoming connections.

Is your network set to be a public one in Windows?

It's set to public. Strange how I can copy paste from the secondary to the primary.

I tried every combination and no clipboard. 

I have this same problem. Does Multiplicity not log anywhere? I saw someone asking in a different thread and the official replies just ignored the question three different times.

Copy on Primary (W10), paste on secondary (W11), the app I'm trying to paste to locks up for a few seconds and then nothing is pasted. Firewall doesn't seem to be logging any blocked connections (I have the firewall logs but no logs report blocking on port 3056x).

The reverse (Secondary to Primary) works fine.

RR, if everyone had this issue, we would be in real trouble no matter the year.  It is extremely rare and almost always a firewall issue; never not seen it be otherwise...  Please see:

https://support.stardock.com/space/SHC/1287454780/Multiplicity+Drag-drop+Copy-Paste+Issues

Sean Drohan
Stardock Product Lifecycle Manager

Did you still run the port fix bat file, which is there in the support article, on both PCs (a reboot of each is not required, but perhaps a good option if it does not initially work)?

Are you certain that both MP installs are the exact same version?

Sean Drohan
Stardock Product Lifecycle Manager

Multiplicity does not tend to log errors because connections can be tested from within the config UI where it displays a message letting you know what the issue is.  Logging could produce very large logs quite quickly given the amount of traffic MP sends in certain situations and how often it will retry connections.  A connection dropping isn't an unusual thing for MP for example as computers get rebooted.

However the config UI test only applies to connections from the primary->secondary.

The issue here is a secondary->primary connection.

When Multiplicity does a clipboard copy, the owner of the clipboard contents is asked for the data.  In this case it is your primary being asked by the secondary.  So a connection is made between them.  The most common problem here is when someone has a PC set with their network set to public in Windows and so this connection is blocked.

Is there an approximately 4 second pause before the connection fails?  If so the problem is a failed to connect problem.  If it is quick then it will be a security issue.

From a command prompt on the primary run "netstat -an | find "LISTENING" "  also do the same on the secondary.  You should see a series of listens for ports 30564, 5, 7, 8 and 9.  This should be the same on both.  

If a telnet to that port fails from the secondary to the primary then it points to a networking level issue such as a VPN running on the secondary thats blocking local outgoing connections.

The other thing is how are the machines connected, is there just one network adapter per machine or does one have more than one interface active?

The other thing is to make sure the clipboard port hasn't been changed on one computer.  This would show up as different ports listening on both ends (they should match)