Working on a Project Server project we discovered it's quite easy to get issues with data integrity when users start retracting and deleting timesheets. After discussing the issue with Microsoft they gave us a workaround which solves our problem.

When you want to use the PSI interface you need to have a PSContextInfo Class. Inside a Project Server Event this will be provided so you don’t need to worry about it’s contents. But outside the Event you will need to create one from scratch.