mod_security is not installed on a ispconfig 3 system by default. So if you installed it manually and the designer told you to install a specific rule, then you should try that. If the designer ahs not send you the rule, ask him to send you the rule.
But I dont think that a 500er error is caused by mod_security. Instead you should take a look into the error.log of the website to see what the reason for the errormessage is.