Download all Submissions Fails

I am using Moodle 2.8.6+ and I cannot download all assignment submission.  If I use Internet Explorer I receive ad HTTP 500 error.  If I use Chrome or Firefox I just get a blank screen.  I believe that there are too many or too large of submissions for Moodle to back up.  Does anyone have any suggestions?  Any help would be appreciated.

Thanks, Diane