"Could not determine user from environment." error

I’ve hit the wall on a WordPress 4.3 error: “Could not determine user from environment.” and need a help on how to fix it.

A little background: there are two websites on one Hostmonster account (no sub-domains). When upgrading Website 1 to WordPress 4.3, the installation was botched and caused Website 2 (which was NOT being upgraded) to be disconnected from its database and to be linked to Website 1’s database. So both Website 1 and 2 were reading from the same database. I got Website 2 re-routed back to the right database, but the error “Could not determine user from environment” persists. The error is visible on the front end of the site as well as the backend WordPress dashboard. There seems to be a lot of sites with this error on the web, but no one seems to know the cause or solution for fixing it.

Things I’ve ruled out and other things I’ve tried:

– It’s not a plugin conflict because I deleted all plugins and the error persists.
– It is not the theme because if you switch to any of the WordPress default themes, the error is still there.
– I’ve tried replacing the Core WordPress files and reinstalling WordPress.
– Checked for malware – there is none.
– Checked for errors that may have been hard coded into post/page content.
– Ran the site through the W3C Validator Service – which yielded no results.
– The Config.php debug mode is set to false.
– I’ve never used the WordPress Multi-site plugin.
– Looked on WordPress Support Forum solutions with no result.

I wonder if the PHP configuration for the Apache Server got messed up, or if this is an error caused by a messed-up database. In the little bit of documentation on the error that I could find, they are linking this error to a DNS mapping issue or a cPanal error.

Any help would be much appreciated; feel free to ask questions. Thanks so much!