We were able to resolve the issue. The WRC noticed an error in the configuration. As it turned out, the agent's address was incorrectly specified on this server. Instead of a locally installed agent, there was an arbiter address causing this error. In the mirror configuration, I changed the agent address for this server and everything worked fine.

Thanks for the answer. But the ISCAgent service is installed and running. The ISCAgent service was the first thing I checked. Restarting the service does nothing.

>> [Utility.Event] Failed InstanceLookup (agent probably not running): instance not found 
The WRC says that this error indicates a problem with the locally installed agent.