API Policy Manager connecting to a Gateway cluster of any version.
The caveat here is that the host information entered into the Policy Manager login screen is to a Load Balancer that is distributing the connection attempt to any of the Gateways in the cluster.
This message in the Gateway ssg log will imply that reuse of a logon cookie for a session is not possible as it has expired and the logon could be from policy manager, web based policy manager or from script.
Another possibility is if someone else is working in parallel with you using a web based policy manager and trying to click on options even after the session is expired, possible that the error is not from your session.
com.l7tech.server.admin.AdminSessionManager: Admin session/cookie not found: <not shown>
In this instance since a Load Balancer is 'front-ending' the Policy Manager, the traffic is getting round-robin between the underlying two or more cluster gateway hosts, and in no time when the traffic hits the second host it comes back with exception saying no session cookies were found.