All seems to be ok, I have Unity 5.0 working smoothly with PM 1.7.8.2 and I'm building to Android fine. When I switch to WebGL build I get the old..
"Plugin 'PlayMaker.dll' is used from several locations
Assets/Plugins/PlayMaker/WebGL/PlayMaker.dll would be copied to <PluginPath>/PlayMaker.dll
Please fix plugin settings and try again."
I am also using the Fast Platform Switch asset which basically caches the settings for each build you require as a quick alternative to switching platforms inside the Unity Build window. Could this be a cause?
I've run the auto update and as it builds ok to Android it seems to be working well, any ideas on the WebGL build failure?