I had the same issue with the "default" login credentials ("administrator", blank password). It worked immediately after reset, but I noticed on the administration page that the password had dots in it (like masked characters), but since I hadn't changed anything from the defaults I ignored it. When I logged out and later tried to login it wouldn't take the blank password. So apparently simply entering the administration page populates the password field with something. I did another reset and immediately put my own password in before logging out and haven't had any issues since.