Hi there, I know this thread is old but I thought this might be useful info. I just had the same problem described above, and I solved it by clearing browser cookies.
I’d been trying something out, and had logged in GM in one tab and a player in another tab. I forget exactly the route I took to the blank setup screen, but my theory is that I was viewing the setup screen as a player rather than the GM, with no way to switch user. This may not at all be the way that Forge works and might be nonsense (once a world is closed, there may be no concept of player and GM logins), but me being logged in with multiple players in the same browser leading up to the issue I thought might at least be useful info to share to help track down the problem.