I followed the transaction in the database user table; after the user clicks on the link from the forum admin to activate the new account, usergroup 10 (non-existent) is being assigned to the user rather than usergroup 4 (Users Awaiting Moderation).
The reCAPTCHA v2 is a mod from this site. I will disable it and see if the control panel home page issue goes away. I will also repost this question in modifications. I was hopeful that there was a programmatic solution to the problem because the native registration function is spammer Swiss cheese...