I looked into that and didn't see any conflicting processes at the report run time. I did do some other research and found that it may be failing looking up the user SID in Active Directory at run time. Also, the report owner owns most of the reports on the server and I found a setting in the RSConfig (Maximum Simultaneous Report Jobs) that limits concurrent report executions to 20 which might be a cause as well. I'm going to increase the config value to 40 and see if that helps.