I’ve removed them and re-added, after deleting the cache and cookies and the same error occurs. When I go to site configuration both show as Succeeded for the Direct method of connection and I can see users, comments etc. I just can’t log in via the dashboard link. It’s quite bizarre.
I’ve just realised the affected sites all have a physical 2FA device for login whilst the others use an phone based authenticator. Not sure why I didn’t make the connection sooner, it’s been driving me crazy trying to figure out what the issue was!