MyBB Community Forums

I don't know how to perform a search for this, so weird. Forum has been working fine so far, no issues, latest version. This morning I log in as admin at .../forum/admin fine. It does not matter what I click on, I get sent to the login screen, and it will not accept my login credentials. If I strip off whatever is after .../forum/admin/, I can log in fine! So I can log in, I just can't do anything. 
*Followup*
When I log in with Firefox, the second login attempt does generate an error "Invalid administration session". Finally, something I can search for! I have cleared my cookies in all three browsers but no joy so far. The path to the site is www.americasguardians.org/forum. This is what is  in the config.php file, which matches what the docs say it should be:

$settings['cookiedomain'] = ".americasguardians.org";
$settings['cookiepath'] = "/forum/";
$settings['cookieprefix'] = "";

If I do figure it out, I will post here that I did so!

check if the login to admin panel works without www (# link)

(2018-02-18, 04:17 PM).m. Wrote: [ -> ]check if the login to admin panel works without www (# link)

That is actually how I normally access it. Same result with or without.

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I should specify that the answer is no, using "wwww" or not using it results in the same failure when I try to actually access anything after logging into the admin root.

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In frustration, I once again logged in at the  .../forum/admin location. This time however, I went to Configure via a right-click and selecting Open in a New Tab. And it opened! But now it doesn't. No consistency, sometimes it does, sometimes it doesn't. Frankly, that makes it worse to troubleshoot. I'm guessing I need the full troubleshooting process here.