I recently had a customer where on all XenApp 6 Workers an Error with the EventID 602 from the Source PrintService was logged in the EventViewer during the User Logon Process. This only seemed to happen when using Session-based Printers in the XenApp Session.
After a bit of Research I found a Microsoft KnowledgeBase Article with an Hotfix which fixed the Problem: http://support.microsoft.com/kb/2457866
Note: This Hotfix is NOT included in the Windows Server 2008 R2 Service Pack 1. I have only tested this on XenApp 6, but this should also apply to XenApp 6.5 Systems.