Whoa, this is a big bug.
Gliffy Version 0.3.4
Gliffy is the ONLY plugin I have installed, and if ANY USER OF ANY LEVEL (from subscribers to admin) other than the original admin tries to log in, this is what you get:
The website encountered an error while retrieving http://your-site-name.com/wp-login.php. It may be down for maintenance or configured incorrectly.
Here are some suggestions:
Reload this webpage later.
HTTP Error 500 (Internal Server Error): An unexpected condition was encountered while the server was attempting to fulfill the request.
Deactivate the plugin, and the problem goes away.