You didn't describe how you tested (which is usually a good idea), but, if your client is using the Microsoft Internet Explorer web browser, you may want to check some of your client IE settings, such as:
Internet Options/Advanced/Enable Integrated Windows Authentication
Also, you may...
(Ok, I fought javascript & cookie issues of this website for long enough to be able to actually post this, so here goes.)
See the T-SQL command sp_dbcmptlevel
and web reference may be found here
http://msdn2.microsoft.com/en-us/library/ms178653.aspx