Can't sign in to itunes, can't authorise

I have two PC's. One runs the current version of itunes. One runs an older version of itunes, which conveniently means the kids can't buy stuff in the store, but can still watch all the DVD's we buy or rent through the up-to-date version.

Recently, apple support got me to log out of all devices, incl itunes to resolve an issue with my ID. But now I can't log in to the old version of itunes.

I enter the same ID and password as the new PC, but nothing happens. When I try to authorise, same thing. Literally nothing happens.

I know this version was working up until about June or July '22, and nothing else has changed.

This means the kids can't watch the movies I've bought since then, and is pretty frustrating.

Is there a fix? I've tried uninstalling, restarting, reinstalling, etc. If I install the latest version it will work fine too.

Can anyone else with around version 10.6 still log out and back in?

Thanks team.