Thanks Kyle.
I just tried to register a new account, and upon completing the CB registration page, prior to the payment page, I received the following error:
Column 'checked_out' cannot be null
I was unable to advance at this point.
The site created my Joomla user account, with a strange last visit date:
-0001-11-29 18:42:28
But no CB user account.
My last CB user was added Jul 12, 2024 and beginning Jul. 13 - 12 new users have tried to register but I assume encountered the same problem, so only exist in the Joomla user database.
I don't recall if I updated Joomla and/or CB at this time, but suspect either may have caused the issue.
Ed