When using Project Server 2010, you may run into the issue where a user submits a timesheet for approval and produces the following error:
“An error occurred while communicating with the server. Check connectivity with your administrator to determine whether further action is necessary.”
In the case I ran across, it was related to the user booking time against a task that was past the finish/end date listed in the Project Schedule. To resolve the user needs to submit their time on a valid task that is still within the current reporting timeframe, or have the Project Manager update the task in questions so that the finish/end dates are at or beyond the submission period and then republish the project. After that, the user should be able to go in and submit their timesheet with success.