My client encountered this error with historic/migrated timesheets after I upgraded them from Project Server 2007 to 2013. They were able to see historic timesheets for employees under ‘approvals’, however when they click on ‘my timesheet’ to drill into the details, they ran into the following error:

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Workaround:

If you go to PWA Settings -> Timesheet settings and defaults and uncheck “Single entry mode”, the timesheets will load fine. However, if you uncheck the “Single entry mode”, you will have to separately enter time for Timesheet and Tasks, which is least desirable.

This seems like a bug in the product.

Bottom line:

You can keep the “Single entry mode” checked and you will NOT be able to see old timesheets that came over from PS 2007 environment. However, you can still build and run report against old timesheets because data is upgraded and available but is not visible unless the setting is unchecked.

Reference: My colleague has encountered similar error with his client that had a different cause and solution, check it out here