The z/OS monitoring went offline after updating the zRemote ActiveGate. Why is that happening?
There are many possible reasons why the z/OS monitoring went offline. The first troubleshooting step you need to do is to check the versions.
As stated in this article, zRemote, zDC/zLocal, and the CICS/IMS Code modules work hand in hand. If you update one component, you must update the others. Else, they will be incompatible with each other.
As best practice, it is recommended that the z/OS monitoring components should be on the same version as the zRemote AG or at least 3 versions lower.