To fix the error 'Logging Week - This week is logged out. But the log is invalid. Please contact System Administrator' - complete the following steps:
Open Timesheet Administration.
Select the correct week of the month.
On the right-hand side of the Timesheet Administration screen, check if the user has a red or white padlock: Red colour means it has an invalid log, and the White colour represents timesheets currently open.
Right-click on the red lock and select Remove Timesheet Log from the context menu.
This will remove the invalid log, and the user will be able to access the timesheet again for that specific week.