CWMDB0090E Another monitor model is being debugged on the specified monitor server. Stop the other debugging session before starting a new one.

Explanation
The debugger can only debug one monitor model at a time.
User response
In the Debug perspective, stop the current debugging session by clicking Terminate  . If you want to completely remove all terminated debugging sessions, click Remove All Terminated Launches  . Restart the debugger for the new monitor model.