Just a quick post to get this into the search engines for the next person.
If you have an Administrative Task in Project Server 2010 that has either any of the following characters in the task name you cannot manually add that task to a timesheet.
There are likely others but these are the 3 I know of for sure.
When you add one of these tasks to a timesheet you get the error “An unknown error has occurred” as shown below.
The odd thing is that you can set one of these tasks to “Always Display” and the task will display with no problem but it cannot be manually added to the timesheet.