by Michael Haskell.
Davo,
It is the standard PDF annotation feature in Moodle 2.6.5 not the PDF plugin which we have never used. I also just timed it and the 500 error is thrown exactly 30 seconds after clicking on the grade link. I've been able to replicate it on another server. I'm going to upgrade from gs 8.70 to the 9 series if possible and see if that fixes it.