When I had that problem it was due to the ezsession database table becoming corrupted - so I just deleted that table and created a fresh empty table from the setup sql file (you have to find the relevent part).
Users could also not sign up for accounts when I had this problem - do you have that symptom as well - or is it just logging into the admin site which is causing problems?
not even the users can login. i already tried to empty the ezsession table but there was effect.. :-/ i think that this happened due to some changes in php-module or in apache. but i can't ask the admin and the phpinfo() doesnt show any clear information about the used version of apache.
