You run smart response rule for Email Quarantine Connect FlexResponse which shows red banner "plugin specified by the response rule is not available for invocation".
Incident history and enforce localhost logs shows below error.
FlexResponse Action Failed [Email Quarantine Connect Approve Action] failed with message: java.io.IOException: Unable to tunnel through proxy. Proxy returns "HTTP/1.1 403 Forbidden"
Release : 15.X
Component : Email Quarantine Connect FlexResponse
Non proxy host list property com.vontu.enforce.nonproxy.hosts was not configured in Protect.properties file
Add below lines with SMG or LB hostname to protect.properties file on enforce located at <Install_Dir>:\Program Files\Symantec\DataLossPrevention\EnforceServer\<DLP_Version>\Protect\configcom.vontu.server.architecture = x64
com.vontu.enforce.nonproxy.hosts = <host_name>