I just took the plunge and upgraded from MySQL 5.6 to MariaDB 10.0. The upgrade went smoothly. I tweaked a couple of settings in /etc/my.cnf, but when I restarted MySQL, I got the error above. I used phpMyAdmin to run repair table on the mysql/event table, which was empty, and after that I could restart MySQL without any errors. I have no idea why, but I thought it might help others.