This means the secondary is unable to connect into the primary for the clipboard data.

Typically this would be a firewall issue on the primary or the use of some sort of VPN tool on the secondary that is messing with packets.

For example Tailscale will cause this.

Any additional thoughts?

Does it detect the clipboard contents and offer paste?

Typically this would be a problem with firewall settings.  The computer you are doing the paste from has to be able to connect to the other computer.  So it would be a firewall issue on the computer which is the source of the clipboard data.

I updated to Windows 11 2025H2 and still facing the issue. I however agree that it is related to firewall. A refresh on my problem: I can copy from the secondary to the primary, but not the other way around. Clipboard history, and hence cloud clipboard are off.

I am able to work around this issue if I turn off the firewall on the primary. I have tried the MPPortFix a few times, even manually deleted all the Multiplicty rules manually from the firewall settings at some point (the rules are back in there now). Hence, I conclude there is something else the secondary is trying to access from the primary and there is some other rule preventing it. Can you send me some suggestions on what other firewall rules to look into? Also should I be looking into Inbound vs Outbound rules?

Edit: I ended up restoring the defaults on the firewall of the primary, re-running the MPPortFix, and all problems went away. I can now copy-paste to-from both machines.