Thanks for your reply.

The fire wall is indeed the culprit.
However, I noticed that my win10 were set as private network while win11 was set as public. (I followed Microsoft recommendation to chose Public).
I changed the network profile on win11 from Public to Private, assuming that the lesser restrictions on private network will allow clipboard and file sharing, but it did not.

I tried to use "Allow an app through firewall" (found on the Firewall & network protection tab), but neither Stardock nor Multiplicity appear in the pre-populated list, and I could not see an option to add an app manually.

I am looking for a solution which does not involve compromising my computer security.
Turning off the Firewall for my Private network is not an option.
If this is the only solution, I will kiss Multiplicity goodbye.

There must be a better and secure way to handle it.

By the way, win10 firewall is on for all networks and it allows the sharing. See pic below

Bingo!!!

Neil, thank you very much!

When I disabled the firewall earlier, the clipboard/file sharing worked immediately, without having to reboot, so I wrongly assumed that it was not necessary. My bad.
The reboot was not necessary in both cases: when my Win11 was set as Public network, as well as after I changed it manually to a Private one.

Following your advice, I realized why it was necessary. Upon reboot, Windows asked me if I want to allow access to the software, with granular permission to either or both network types.  As per your instructions, I approved access to the private network only.

I was curious to see if Stardock / Multiplicity was added to the list of "Allow an app through firewall". It was indeed.

Now everything works.

Thanks again!

Just for the record.  Public Education...
I want to share new information (at least for me).

I previously wrote:

 -- I tried to use "Allow an app through firewall" (found on the Firewall & network protection tab), but neither Stardock nor Multiplicity appear in the pre-populated list, and I could not see an option to add an app manually" --

Well, it is possible to manually add an app to the exception list.
You need to sign in as Admin by clicking Change settings and then click on Allow another app...