I checked into the mod_security logs but wasn't able to confirm this as the cause; there's a large number of hits present for your site in those logs, but all of them I could find were known security exploit filters we use to help keep our WP users secure. Unfortunately for safety we cannot disable these filters even on an account-specific basis, as the types of attacks being attempted would, if successful, result in a complete take-over of the account, allowing it to execute any commands the attacker wishes including but not limited to sending spam, running malicious processes to try and break into the server itself, and running arbitrary code on a victim's browser (in the case of XSS vulnerabilities).
One issue I did notice is that the permissions on your .htaccess files and on wp-admin/post.php aren't correct; files should only ever be 644, while some of these are 777. As another security measure, files with 777 permissions can and usually will throw a 403 error - 777 allows any user on the server to run and execute the file, as opposed to just you (the file's owner) and/or root (admins). While it's unlikely this would be exploitable with our setup, we do have that in place as an additional security check.
The best recommendation I can make is to reset the permissions to 644 for files and 755 for folders, then see if the error continues. If it does, reply back with a description of what was being done, what browser was in use, and the time you tried it so we can try and investigate further to see why it's causing a mod_security hit, or if it's being caused by that at all.
