Sorry for the delayed reply.
Thank you all Divyanshu Srivastava, Srinivasan Vinayagam, S Sriram for the quick response.
The problem was in another building far away from me.
The actual reason was, that after reinstalling the PC, the local sysadmin did not ajdust the domain user accordingly. I don't have any more details, but it is OK now!
Case closed!
Thank you all again!