Okay, I have a few questions for a getting a better view of the situation (not sure actually if they could be relevant):

- Is the user account running the client application a member of the workstation domain or the server domain? (Or perhaps a third domain?)

- Is the client application a normal windows application or a web site?

- How does the client application connect to SQL Server? Directly or via a server application?

- What kind of user account does the client application uses to connect to SQL Server? A domain account, local user account (in case the server application or web site runs on the same server as SQL Server), or a SQL user account?

- In the case of a domain account, which authentication scheme is used, NTLM or Kerberos? You can look this up in sys.dm_exec_connections (http://msdn.microsoft.com/en-us/library/ms181509.aspx)