We have the same environment and same problem. Quovadx Support says it will be fixed in 5.6.
Some of the process based NetMonitor commands (like purge caches and reload) are broken. The GUI passes all of the thread names in the site to the process (instead of just the threads belonging to the process). The process recognizes it does not own the requested threads and errors the command.
The commands seem to work OK if you request them from a single thread instead of from a process.