Thanks very much for your reply. I had already disabled all plugins and changed the theme to the Default 2020 without any success.
That said, I installed the Health Check plugin, went into troubleshooting mode and replicated the problem. The only issue I see here is that the plugin only disables the site for me as admin from what you say, but I need to logout in order to test the registration. This resulted in the same issue with being unable to register and the same message appearing.
I then tried registration whilst logged in but did so in a Private Window in Firefox. Same issue. I then tried registering using Google Chrome. Again, same issue.
Cheers,
Ric