Once again, I had some testing with this issue.
Raising max_execution time to 600 brings (notice: always changing lines, where the error occurs)
PHP Fatal error: Maximum execution time of 600 seconds exceeded in /srv/www/moodle/lib/dml/moodle_database.php on line 793, referer: http://hostname/moodle/mod/assign/view.php?id=42&action=grading
I thought it might be an issue of my server configuration, I set up a testing environment with completly fresh Ubuntu Server 12.04, actual updates and plain moodle 2.6. Then I made a backup of the failing course, restored in the testing moodle and... exactly the same error. With other courses (less participants) everything works fine.
I found a workaround with custom reports block to get at least an export of the grading tab, but other hints would be helpful. Do these features (download grading tab and download all submissions) from the assignments menu work for other users with 2000+ course participants and 700+ submissions for an assignment? Isn't 10 Minutes max_execution_time really, really enough for a well equipped server?
Kind regards,
Jan