Upgrade from 5.6 to 2017 - Editor Freezes on Play. Windows & Mac

We upgraded a project from 5.6 to 2017.2.1 and we cannot run the project.

Whenever I press Play the Editor freezes. If we try to build it freezes again when the process starts.

Same thing happens to all our computers. PC or Macs.

Has anyone come across this issue and where you able to tackle it?

Does this also happen with a new project? You may need to reimport your assets.