It died trying to investigate this.
I had to rush off, so couldn't look into it any more, but when I tried to look at the Admin Security log, it basically used up all mysql time.
I ended up restarting mysql to fix it.
When I used a user agent changer in firefox and set it to "Mozilla/5.0+(compatible; UptimeRobot/2.0; www.uptimerobot.com/)", I also get the 500 error.
Server specs (Server OS / Apache / MySQL / PHP / DragonflyCMS):
Joined: Apr 19, 2006
Location: Sydney, Australia