Ou domain scan fails with general failure error

When I try to scan computers from domain, I get general failure error in domain scan wizard. This error is present when I put domain controller name in settings page. Account is Administrator account with all rights. If I don't put DC name, I will get list of all computers without any problem, the domain wizard will not even start. This is a great problem, as I was waiting eagerly for a possibility to choose OU's as our domain is composed of three different companys, and I don't want to scan their computers. There is no firewall between server where ServiceDesk 5 is running and DC. Only thing that comes to my mind is that we are in mixed enviroment, before passing definitely on 2003 native mode. I am waiting for your feedback.