Apr 12, 2015 05:21 AM|smirnov|LINK
There is a dedicated forum for AD and LDAP where you could find many similar threads, ex. http://forums.asp.net/t/1971340.aspx?Check+if+User+is+part+of+AD+Group
Under what identity is your process running on the server? Most likely, that user does not have the correct rights to access your Active Directory.
The membership in the Windows Authorization Access Group is required to execute GetAuthorizationGroups.
See the following article: http://support.microsoft.com/kb/331951
Also you can try
PrincipalSearchResult<Principal> groups = up.GetGroups();
Hope this helps.