PWA, timesheet errors

In PWA2007, after adding or deleting a line in a timesheet that comes from a
project plan and clicking ‘Save’, I get the error “While communicating with
the Project Server an error occured. Check connectivity with your
administrator to determine if further action is necessary”.

The changes made to the timesheet are then lost.

The same problem does not occur when lines which are not from a project plan.

What can I do to resolve or work around this problem?