APM: Why can't get the session.user.sessionid in the ACCESS_ACL_ALLOWED?

What is the error message that you get exactly?

Also, you should not use mysession across events like this because these variables have a per TCP flow context. If, for some reason (HTTP), the TCP connection gets recreated, mysession will no longer be in-scope because ACCESS_POLICY_COMPLETED will not fire for an existing session and ACCESS_ACL_ALLOWED will fail with a TCL runtime error because mysession is no longer present. Since this value is cached anyway, just re-run [ACCESS::session data get session.user.sessionid] in each event.