Stuck on Compiling Scripts popup

I am using 5.5.2f1 and when I try to upgrade to 5.6.3f1 or to 2017.1.0f3 it always comes up with the "compiling scripts" dialog and hangs.

The log files shows it is starting the assemble build but go no further than that. The process Unity Editor is running and constantly adding to its memory. The Mono process is fixed in size. There are no hidden dialogs that I can uncover via al-tab.