Haven't seen that error before, but it's most likely due to some security setting on your server. According to this stackoverflow thread you may be able to solve it by adding these settings to your .htaccess file:
I think your best bet might be to contact your web host and see what they have to say about it. I haven't seen anyone else with this issue so it seems like excessive security settings are to blame here. However I noticed this thread has quite a lot of views so before I jump to conclusions I just want to make sure no one else is having this problem, because I may consider changing the login code if that's the case.