Can only build DEVELOPMENT with WebGL

Hi I have a multiplayer (Photon server) fps project which started way before Unity5 and I'm trying to get it to work with WebGL

So far I can build to development, it takes app. 15 minutes, but it works.

I can't build if I turn of Development, even when I let it run for the entire night. It hangs at the last step: "Converting managed assemblies to C++"

I have not been able to find anyone with similar issue and I receive no errors.

Any idea on what might be wrong? What I can look into?