Open up a support case and provide the technical support file to TAC. Without all of the log files the only thing anyone on here can do is guess at what the issue could potentially be, TAC needs to look at all of the logs and determine if they can pinpoint what caused pan_task to over-run. You could be running into PAN-133883 but this seems unrelated.
Just as an aside, when you are running early releases of a new major release, you should expect to run into issues like this. If you are running a new release early in its life cycle, I would highly recommend staying as current as possible once you've validated new releases in your Lab environment. Personally right now, I'd recommend 9.1.2 to anyone on the 9.1 branch.