I recently saw this error on Project Server 2010 when trying to access Tasks under My Work:

This was a little tricky to tie down but it was happening to everyine that had been assigned a task in a particular project.  I ensured the resources in the project were properly assigned in the pool (from the enterprise) via Project Professional 2010 but that did not solve it.

In the end I took a copy of the project and saved it locally, then completely removed the project on the server.  I then opened the local copy and republished – this seemed to solve the problem and we can all now access our tasks.