Okay I have replicated the error and gotten rid of it with the steps I listed before. I did not import the PUN package. I will list the steps in the order I did them starting a new project from scratch in unity 5.4.
1. Import and Install Playmaker 1.8.3
2. Import Photon 1.75
3. Go to this link
https://github.com/jeanfabre/PlayMaker--ExitGames--PhotonUnityNetworking_U5 Click Clone and Open in Desktop. Choose a folder I put it in a new folder on my desktop called playmakerJIN.
4. I opened up the newly cloned folders and selected 5 folders within the Assets folder. PlayMaker Custom Actions, PlayMaker Custom Samples, PlayMaker PUN, PlayMaker Utils, Plugins.
5. I copy and pasted them into my new project called playmakerJIN Assets folder. Overwriting the current files.
6. I then opened up the DemoWorker scene within the included Custom Samples folder from the GIT clone.
7. setup my photon cloud.
8. clicked on Menu in the Hierarchy, clicked on connect, unchecked Connect to Best Server.
9. Hit Play all is dandy, ignore any errors you may get at first from the copy/paste action from meta files and such.
That is my actual experience for fixing the problem that I also had. Hope it helps. I'll make a video if you need also.