It looks like this does not get logged. (I really should get logged. I created a bug report.)
You may be able to work it out from other server logs. E.g. the time is probably stored in the 'quiz_overview_regrades' database table, and your web server logs or other Moodle lots may show who was acessing the quiz at that time.
You may be able to work it out from other server logs. E.g. the time is probably stored in the 'quiz_overview_regrades' database table, and your web server logs or other Moodle lots may show who was acessing the quiz at that time.