CSRF error on login

@php_rocs:
Thanks for the info on the versions. I have downloaded version 3.1.9 and will ask my sysadmin to upgrade our PHP version (on our dev server first of course ). I knew PHP 4 had reached its end-of-life but not any versions of PHP 5. But I knew they were up to version 7 (just checked on PHP.net and the latest stable release is v7.2.6) so I should probably get that upgraded anyway.

Now if you think it should be logging something, I will try changing the logging settings to log info and debug messages. In fact, I think you hit the nail on the head with logging. Maybe I can throw in a bunch of log_message calls throughout the code and see what comes out.

And yes, I'm sure I did everything that was in the documentation; if there is some step that's NOT in the documentation, I haven't done that (not meaning to sound rude - just saying that cuz I've actually seen frameworks that leave out important details in their docs). Is there something about the CSRF protection feature that isn't there, that I should know about?

@InsiteFX:
Thanks for the suggestion. I couldn't find "internet options" (Isn't that an IE menu?) but I did check my cookie settings and they are not blocked. That was the only setting that I could find (to block or not to block, that is the question ).

Now as far as the F12 developer tools are concerned, what specifically should I be looking for? The only new info I found there is that the response code is 403 Forbidden. The CSRF cookie is there, and the response text is just the HTML of the error page.

@both: Thanks for all your ideas, questions and suggestions! I really appreciate the help.