BuddyPress.org

I looked hard to find a solution for this problem, but I can’t find it. Hope somebody around can help. The site involved is http://www.degravinutrecht.nl

I did a clean WordPress install (3.5.1) and a clean BuddyPress install (1.7.1). Everything works smooth, but when I try to log in on the front end, I get the following error:

Bad Request

Your browser sent a request that this server could not understand.
Apache/2.2.16 (Debian) Server at http://www.degravinutrecht.nl Port 80

What I’ve tried so far with no success:
– delete cookies
– try different browsers, different computers (mac/windows)
– disable all plugins / themes
– reinstall files (both BuddyPress and WordPress)
– register as a new user and confirm by clicking the link in the confirmation e-mail
– reset the permalinks
– making the register and activate pages, removing them and make them again
– selecting and deselecting the ‘anyone can register’ checkbox under Settings.

Even if I leave the username and password fields empty, I get the same HTTP 400 error (instead of being redirected and given the message I need to fill in a username and password).

Everything else on the site works perfectly. Also, when I install a login plugin, I am able to login on the front end, no problem. This however does give users also access to the WordPress backend, which is not my intention.

To go short: why do I get a 400 Bad Request error when trying to login at the front end of my website?

Thanks for your help!