Can't access wp-admin

No problems on my site hosted by ww.wp.xz.cn. I have three other sites on my own server, and I am unable to login to wp-admin on any of them. Get to the Login page, enter valid (at least it was valid last time I tried it) userid and password. Site returns a blank page with “Not Acceptable” in the upper left corner. Go back to login page and try to ask for a password reset and get the same thing.

I’ve tried cleaning out cache on my browser (Chrome) to no avail.

I tried using Safari, and was able to login ONCE to one of the websites. As soon as I attempted to log out, got “Not Acceptable” again. Now not able to login at all using Safari, even after clearing Safari cache.

I’ll have to resort to FTP to try to fix this – not a fun undertaking.

Any ideas would be appreciated.

P.S. The WordPress version listed (3.5.1) is the lowest one for the three websites. The other two are running later versions (but not the new 4.0)