At first look I would say this is not related to the dynaTraceMonitor error. Which PurePath did you find this log message?
The first thing I would recommend is running UEM Health Check in the Start Center. That will inevitably make you revise your Application and User Experience configurations (checking for URLs, places of injection, etc.).
How did you get the evidence that the signal is receiving a 404?
I'd definitely run the UEM Health Check.
It could be that the monitor signal is arriving at a JVM where UEM is not installed or active.
I'm assuming you changed the UEM configuration in the system profile so that the monitor address is /dynatrace/dynaTraceMonitor.
Anyway, the health check should tell you.