Windows XP Password after locking computer does not work

Hi,

I have following problem.

When the user logs into his PC (on the domain) it works fine. He can enter
his password and then work. But when he locks his computer and wants to enter
it again by hitting ctrl alt del the system does not let him in. It tells him
that the password is wrong.

he needs to reboot and then he can log in again fine.

can anybody help?

I already updated group policys, removed and added the pc back to the domain.