This problem and solution is surprisingly quite “undocumented”. When you switch to Revit Server, the normal Worksharing Monitor for Revit will not work anymore.
However, you can become a part of Autodesk Project Bluestreak (also known as Activity Stream for Revit). Basically, you join Bluestreak, install Bluestreak Desktop, and install Autodesk Revit Activity Stream.
Definitely worth a look if you are using Revit Server.
The Revit Jedi: Workshare Monitoring Vs Bluestreak… the Smackdown!