Login fails, loads /wp-login.php/

A site I’m running is having problems. With pretty permalinks enabled, after login I get an “archive not found page”, as WP tries to load a page slug “wp-login-php”. If I set default permalink structure, WP tries to load http://www.example.com/wp-login.php/ and fails.

As far as I can tell, this problem just appeared. I had problems before with couple of posts – trying to update the posts would cause a similar problem. However, that happened with just three or four posts, so I didn’t mind. Now I mind as I can’t even log in to the site.

I’ve tried:
– setting .htaccess to default
– re-uploading WordPress files
– disabling mod_sec in .htaccess

Since I don’t think this has anything to do with what I’ve done – as far as I can tell, there are no plugin or theme changes involved (I’m not the sole admin, so I can’t be 100% sure, but I’m 99% sure) – I’m guessing this might be caused by some change in server settings.

But what could it be? It would help a lot if I could ask the domain guys to ask the host to fix a particular problem – I can’t talk with the host support myself. Any ideas?