This may be a plugin or theme conflict. Please attempt to disable all plugins, and switch to the default Twenty Twenty theme. If the problem goes away, enable them one by one to identify the source of the problem.
If you can install plugins, install Health Check. On the troubleshooting tab, you can click the button to disable all plugins and change the theme for you, while you’re still logged in, without affecting normal visitors to your site.
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.
I have found the answer!
This site was a new WordPress site that I imported from an old SMF forum. When it came across into wpFORO, it brought the User IDs with it into the User table in mySQL.
Although AUTOINCREMENT was set in the table, it didn’t know from which number to add new user IDs.
The instructions on how to do this were found at this post….
I certainly hope this saves someone else the agony that I have been going through.