Hi,
I am really sorry, I can't replicate this, create a new project, installed PlayMaker, Photon from the asset store, then installed PlayMaker Photon support from the ecosystem ( VERSION5, there is a version for Unity 4 as well, watch out), installed the demo, and all works well, I can publish and run the demo with 2 users in the room fine.
I am pretty sure it's because you need to clean up your project, or because ou imported the wrong package.
Else, the next step will be for me to have access to one of your project so that I can see this error and debug it within the project. Pm me.
Bye,
Jean