With all the great feedback and information, I was finally able to track down the bug. It is too long and complicated a story to go into, but what is important is I think I have a fix.
I'm attaching an updated auth.inc.php (public_html/admin/ directory). If you guys would give it a try (overwrite your existing one) and let me know if this solves the problem. If it does, then I will roll out 1.2.1.pl6 with this and a couple of other minor fixes ASAP.
savco - Since your problem is only with FireFox, I think it is something specific to your browser / add-on configuration. If I remember correctly, you told me the error does not happen with other browsers. I've added the IE Tab add-on to my FFv4, but still everything is working as it should....
I just noticed that the error.log is actually empty..not even a single entry is that supposed to happen ?
I would expect there to be something in there if the token check fails....I'll look closer at the source a little later this morning and see if there are instances where the token check could fail and not log an error.
The problem that is fixed with the updated auth.inc.php would affect all users and all browsers. The bug was very specific that it only appears when you have to re-authenticate when entering the admin area.
BTW - the best way to contact me via email is mark AT glfusion DOT org - I still think it would be worth a look around your site to see if there is something that is site specific - but, if you had the same error here when trying to send me a PM, that really makes me think that it might be browser related, not site related.