Update: Version 0.975 resolves this issue. 

Unity 5.6 introduces a new bug by changing the communication between an Unity App and the surrounding website. This will cause problems while loading the java script code of the plugin.

For now the problem can be avoided by using templates which include the java script code before the unity app is started. Simply import the unitypackage files and select the WebGL templates via the WebGL Player settings. You can find the needed files here: WebRTC Network ,  WebRTC Video Chat template.

You can find more information about Unity WebGL templates here (not yet fully up-to-date).