session by database and login error

[eluser]elombashy[/eluser]
hiii

I turned the $config['sess_use_database'] into TRUE and built a table in the database, in order to save cart session, and this worked fine, but then i got another problem in logging-in..
for some reason, turning the "sess_use_database" on, prevents me from logging in the site.. can any one explain this?