Thank you for posting this thread Michael,
I have just installed moodle and got the error 403 msg upon trying to register - thanks to your efforts and you're generosity in sharing the information I was able to just get straight on to Hostgator who fixed the issue across the account...
according to their technician the mod_security rule is whitelisted for my entire account and I should not longer receive that error.
You have saved me hours of trawling through forums etc. trying to pin it down!in appreciation,
Ruth