I use essay in each homework and have never faced this problem. Anyway, it deserves deeper investigation. There could be something broken in your installation, as well as some of the answers may contain characters which produce error when the answers are extracted.
1. Open the grading page and then check the apache log file.
2. Check the text of the ansers in the database. Use phpmyadmin or adminer, for example. Is there something suspicious?
3. Set up new empty course with test students. Does essay work?
4. Copy your quoestion to the test course. Still working fine?
5. Copy the answers of the students. Does this step break something?
I think that without this investigation it is hard to move further.