The issue is reporting something about your proxy not being able to relay requests. Are you testing from behind a proxy? From the note, it seems like you're connection is behind a ISA (Internet Security and Acceleration Server) Proxy, but either your username/password are not configured correctly or the ISA is not configured correctly.
As for the file that's trying to be loaded, that file is only used when two-factor authentication is enabled on your site. That file will check if the user that's trying to authenticate has 2FA setup. If so, then the form will change to show the 'Secret Code' field. If the user doesn't have 2FA setup, then they will be logged in.
Other things to check/try:
* Disable the 2FA plugins on your site. That should make authentication work every time.
* Ensure there are no firewall or .htaccess rules on your server preventing access to the file you linked to
* Check that you are loading your site over www. The file thats being denied is over www as well. If you're trying to load the site over non-www, but see the error message for the www. file, it means your page caching (System - Cache) plugin is causing problems.
I hope that helps,
Alex
Intermittent Login Button Functionality
