I've seen something similar to this during plugin development (i.e. self-induced) and was able to ultimately work-around the issue by clearing the session cookies for the site. Please note that I said "work-around", not solution!

You can see, without any special debuggers, what's going on in the store by installing the Zen Cart Notifier Trace (http://www.zen-cart.com/downloads.php?do=file&id=1114) and then try logging in. Be sure to go back to your admin once you've captured the set of notifiers and turn the Notifier Trace off (Configuration->Logging) because the file can grow very large very quickly ... especially with the condition you're describing.

The plugin creates a file in your /cache folder named notifier_trace.log and contains a list of all the page-level processing that's transpired.