If this blog had a sub-title it would be "Task updates never get to Project Professional - even when they (eventually) say 100% Success". And the error message you might find in the ULS logs would be:
Microsoft.Office.Project.Scheduling.SchedulingException: The engine's event horizon was extended beyond the maximum length of 36500 days. This may be caused by invalid task or assignment data. You can also change the value of the InitialProjectDays scheduling parameter.
This error led us to review the project timescale, and at first glance the Project Information was showing reasonable start and finish dates - from late 2007 to mid 2008. But when looking at the plan the project summary task went back, and back, and back... There was a milestone task 100% complete for 1/4/1984 driving the start date of the project summary. Not sure at this stage how it got in that state - but this makes the scheduling engine work very hard processing status updates (you may see the queue service using LOTS of RAM) and eventually it gives the above error (in my case after 30-40 minutes) but the job appears to have completed OK. However no updates appear in the plan.
The current workaround is just to correct the date, save and publish and then re-submit the time.