Page not found (trying to login as admin)

Hi guys. Today I got a message from a publisher that was unable to login on one of our blogs.
I’ve tried myself… and no one let me go in (on 6 different ones).
I’ve worked with them last week and all was fine.
When I try to login I get the login page, when I insert the login data I get a “Sorry, the page your requested could not be found, or no longer exists.” Reading around I’ve found that could be a plugin or theme problems. I’ve also tried to login with unexistent data and wp responds well telling me there is not that user.
i’ve made a test disabling all (via ftp) but all the same problem.
So what happened?
All the rest of web site runs ok… and I think is not a corrupted file since the problem is on all 6 blogs (every one with its mysql table and space).