UE4: OSP Wwise plugin error regarding the number of OSP mixer used

I followed this documentation to test the Oculus spatializer Wwise plugin. The spatializer seem to be working but in the case of remote connection to Wwise Authoring, I got an error every time once clicking on the "Stop" button in UE4 editor. It said:  "[Oculus] Error: multiple instances of OSP mixer plugin in Master-Mixer hierarchy is not supported, only one is allowed." 

I've checked that I only had one audio bus with OSP mixer plugin under Master Audio Bus. Maybe in my case some other settings were wrong and caused the issue?