Actually, I think this is a remnant of my previous fumbling. The issue began yesterday (I think with the addition of another subdomain) when other users are complaining about: 1- they are seeing 310 Too Many Redirects, or 2- they need to register in order to access the website's content (which is nonsense, I've never set that up).
I've done quite a few things to resolve the issue:
1- Switch to default permalink structure and switch it back
2- Disable a plugin that has to do with user registration (that I never needed, it was set up to fix another issue)
3- Change 'User can register' from '1' to '0'
4- Change all instances of 'www.domain.com' to 'domain.com' (in both the options table and wp-config.php)
5- Install this plugin Permalink Fix & Disable Canonical Redirects Pack
6- change //define( 'NOBLOGREDIRECT', 'www.domain.com' ); to //define( 'NOBLOGREDIRECT', '' );
7- Clear cache (a million times already)
Now, here is the thing. 310 is gone, but there are a lot of remnant of the signup.php problem:
1- Site can be accessed with http://www.domain.com, but not domain.com. If I try to use domain.com, it send me to http://domain.com/wp-signup.php?new=domain.com
2- I can no longer use any of the links on the WP admin bar because they eventually get redirected to http://domain.com/wp-signup.php?new=domain.com. I can only access them using http://www.domain.com
3- Can't get into network admin, regardless of the presence of www. All end up redirected to http://domain.com/wp-signup.php?new=domain.com.
4- Going to any page (such as domain.com/page-name) send me to http://www.domain.com/wp-signup.php?new=domain.com (how funny, just when I thought http://www.domain.com is no longer the problem).
5- Finally, domain.com/wp-signup.php?new=domain.com is a page with errors whereas http://www.domain.com/wp-signup.php?new=domain.com tells me the following information:
Greetings Site Administrator! You are currently allowing “user” registrations [comment: bulls***, I disabled user registration long time ago]. To change or disable registration go to your Options page. You are logged in already. No need to register again! The site you were looking for, http://domaincom.www.domain.com/, does not exist. [comment: where the hell is domaincom.www.domain.com, I don't have it configured ANYWHERE on my site]
So, I'm going to change all of the domain.com back to http://www.domain.com, see if that fixes any problem. If not, I'm out of luck. Sigh, this is my unlucky wednesday.