pgAdmin4 won't start after OS upgrade

Hi all,

I upgraded my laptop yesterday from Debian 9 to 10 (Buster), and now
pgAdmin4 won't start. It tries to start for a while, and then I get a
little dialog saying "An error occurred initialising the pgAdmin server:
Failed to launch the application server, server thread exiting."

I have the pgadmin4-server, pgadmin4-desktop and pgadmin4-doc packages
installed, version 4.25, all installed via apt from the pgAdmin repo.
Removing and reinstalling them makes no difference. I have also deleted
the settings database at ~/.pgadmin/pgadmin4.db, but no difference. The
log file at ~/.pgadmin/pgadmin4.log is empty.

Where else should I be looking? I know nothing about Python, so I don't
know where to start.

Thanks in advance - any help will be greatly appreciated.

Ray.

-- 
Raymond O'Donnell // Galway // Ireland
ray@rodonnell.ie