Have you used the FoF Passport extension in the past? I don't see it in your list of enabled extensions. I'm really curious where that URL comes from on your website if you never used the extension.
If there is no custom code on the forum, then this might be related to a conflict between extensions. You would have to try disabling some extensions to see if there's any difference. Since you have so many extensions it will likely take some time. You can try disabling half and seeing what happens, and repeat. You could try disabling all the extensions related to authentication and theming as those are the most likely to impact the login button.
If you haven't already, try clearing the cache from the admin panel or the command line.
