SSRS Integrated Mode - Scrambling Connections?

Have an odd error that occurs intermittently:

• SSRS Integrated mode (2008R2 on SharePoint 2010)
• Security is Windows Authentication (Kerberos Delegation configured)
• There are data sources for both SQL 2008R2 and SSAS (Multidimensional) 2008R2.
  • The problem definitely occurs with reports that are relational only
  • The problem occurs with reports that have a mixture of relational and SSAS as well

Problem:

• Occasionally a user (let’s call him DOMAIN\user1) runs a query and receives an error message:
• “The permissions granted to user ‘DOMAIN\user2’ are insufficient for performing this operation. (rsAccessDenied)

It "feels" as though when user1 runs the query, the per-user connection for user2 is being used to access the data, resulting in the wrong AD Kerberos ticket being sent to the back-end database.

Has anyone encountered this type of error before?

Rob Kerr SQL Server MVP CTO, BlueGranite