You may notice this error when you enable cPGuard WAF, specifically if you are using RUID2. This is a classic bug with RUID2 handler and an official fix is not yet released. To stop appearing this error message in your logs, please do the following steps.
1. Open file "/etc/apache2/conf.d/modsec/modsec2.user.conf"
2. Add the following line after "Include /etc/cpguard/cpguard_modsec100.conf"
SecRuleRemoveById 400000 400001