I tried to change it back to Built-In stores and again to domain. But it didn't help. Strange was, that the Test Connection button in the setting dialog worked fine. But after it, I am getting the same error message again.
User account is OK, I used my own account as the user for connection to domain as well as for ArcGIS Server user. I am logged with this account in my computer with ArcGIS Server.
I am using Windows 7 Enterprise, but I get the same error on Windows Server 2008 as well.
Thank you for any other suggestion,
Zdenek J.