I logged call to the Thecus official support page also but nobody was answering there. Why Thecus is keeping that page anyway?
As nobody was helping me i found the solution to the issue myself after many long hours of desperite tries. The ADS join finally started to work when i changed the small/capital letters to the same as it was existing on the W3K domain server for server name, domain name and domain realms. Every letter should match the same capital as in Windows. When all letters matched with same capital the join was succesfull. This sounds stupid as with the previous firmware didnt have this problem.
Thecus support is very very poor, i will reconsider buying Thecus products in the future, probably will not buy anymore nothing for my company.