/edit. Just to add a datapoint. While it's not conclusive, all the Miniprotein_relax8 units I'm getting that run long do "complete" and show as valid, even after going 10 hours over. Of these units that run over, many are "seconds" sent to me from other machines that failed to process the WU. My machine is running OSX and completes them fine (beyond running 10hrs over). All the failed machines are windows or linux based. That said, I know Macs make up a small percentage of computers on this project, so I might have just not gotten a resend from a Mac in my small sample.

I've noticed than some of the latest Tasks aren't checkpointing properly, so if you interrupt them they will revert back to the last successful checkpoint.
Next time, just let it run- the default time is 8 hours, and there is a 10 hour watchdog timer in case it's not done within 8 hours. If it's still going after 20hours, then you might want to kill it off.

It sometimes helps to shut down BOINC, then restart BOINC and then the task.

Progress seeming to freeze near the end of a task is often a sign that the task was created with a severe underestimate of how long the task would run.

But, of course, during this holiday period people here expect to demand the creation and issue of sufficient work to serve at least a third of a million tasks per day to the world at large, with no respite.
And the amazing thing is, a fair few do seem to have come down. Maybe my caches aren't quite completely full, but near enough.