I tried uploading
ucp_profile_reg_details.htmlfrom the 2.0 version of the mobile style on my board, and refreshed the templates and purged the cache.
The problem is fixed.
However, it seems any 3.x version of the Artodia: Mobile and SEO style, that particular file will not work, on my local board or my live board.
But, it is now fixed, and that's the main thing.
If it happens again, I will upload the 2.0 template file.
Thanks for your assistance!