Ding ding ding, mod_security has another victim...
Your error log should hold the phraise that pays, if it's something critical to running your forum, you'll need to provide the string to your webhost; if it's something random that you'll never see again; you can usually ignore it.
mod_security is a module which identifies "strings" at a set list of known exploits, most of the time it goes overboard and flags legit scripts as mallicious; keyword targeting at its worse.
