I had this problem after my website was subject to a brute force attack. I had the admin login username details set to "admin" and had the Stop Spammers plug in installed. Every time I used the admin log in, it kept me out. I had to clear the cache in the dreamhost panel, get help from dreamhost Via LiveChat. when that was all reset I signed in, downloaded ADMIN USER NAME CHANGE plug in, changed the admin user name to something else and problem fixed. Took a while, but not sure if this is the same as your problem.