now that's scary enough and I am happy that mod_security blocks that type of request. or should it?
No, it should not as it is not a injection attack.
Thats a false positive in mod security rules. A controlpanel mus be able to send a post request that contains the name of a system path like /etc/, otherwise you would not be able to administer the server. And this has nothing to do with the ajax functions in ispconfig.