Our Crystal Report ODBC's are set up to use Windows NT authentication. I have each user setup as a Login in the SQL Server Management Studio with datareader privleges for the databases involved. This has worked fine for a while now. When a user opens a Crystal Report the Logicity Viewer uses their Windows credentials and displays the report without asking for a password. Recently users have started getting a Database Login popup (see attachted screenshot) requesting login information. Even if I enter valid login info the login attempt fails (signified by another popup message). It's a periodic occurence that seems to be happening more and more frequently. Often the user only needs to wait a few minutes, try again, and then it will work. This occurs on all platforms from 32-bit XP to 64-bit Win 7. I don't know where to begin to troubleshoot this issue. If you have any ideas I would like to hear them.
Some more info about our environment.
Crystal Reports XI
SQL Server 2005
Windows Server 2003 Std