Remote access to Macos 10.15.7 (Catalina) from macOS 12.6 (Monteray)

I am trying to use remote access from my new Macbook Air (macOS 12.6 (Monteray) to my 2012 iMac (Macos 10.15.7 (Catalina)).

Using ssh works fine in both directions. I have set up 'sharing' properly in 'System Preferences' on both machines'. When I go from Catalina -> Monteray, it works fine.

When I try to go from Monteray -> Catalina, I get 'connection failed' and "Make sure Screen Sharing or Remote Management (in the Sharing section of System Preferences) is enabled on the remote computer.

"

As I say, I've set and unset the sharing, and restarted the machines, many times, but this seems a solid problem.

As I say, ssh works fine, so this seems peculiar.

I'd be grateful for any suggestions.