Hi Kemal,
Reproduce the issue and check SU53 for the user in the back end system.
If you are not able to login with the user, go to su53 and check Menu-> Other user and give the name of the user, it will show you the last unsuccessful authorization check for the user.
There you will find out the authorization missing for the user.
Assign yourself or ask the security person to provide the missing authorization.
Hope this helps!!!
-Amit Mahesh