Occurs when a user wants to view the "Site Audit Log" or "View Workflow Reports".
Something that happens when you import a site. The Reporting Features are active, but do not work. I think this is what caused it in anycase. Could be something else.
Deactivating the Reporting Features at Site Collection Level and then running the following command:
stsadm -o activatefeature -name Reporting -url http://servername/sitecollection -force
Seems to fix the problem.