Probably your activity IDs for the task don't start at 0, or are not sequential as Akkadius suggested.
If the Task ID really is 303, then there should be messages in your zone log files detailing why it couldn't load the task, but the above is most likely.
If that isn't the problem, we would need to see the relevant rows from your task and activity table, i.e. the output of:
Code:
select * from tasks where id = 303;
select * from activities where taskid =303;