WordPress MU / users blog not loaded

Hello,

I installed WordPress MU version on a sub domain >>

http://webs.domain.com……

It works perfect

If a user signup a new blog and user are created ok too

BUT the Blog is not working >>

http://username.webs.domain.com…..

what is the problem

Note that the users cannot log into his account too.

always >> page cannot be displayed error on user level..