I ended up uninstalling and reinstalling the sim and starting fresh due to having significant performance issues and slow loading. I uninstalled and reinstalled the sim with most things and settings backed up and oddly ended up having the same issue as you. To clear the hung welcome screen, I ended up deleting the Scenery config file (located in C/Users/user name/AppData/Roaming/Lockheed Martin/Prepar3D v4) and then going into my main P3Dv4 folder and clicking (not removing) the Delete Generated Files file. After that, my sim starts normally and loads up very fast. Since re-install, my P3Dv4 is running better than ever. I am being careful to measure the loadup speed degradation after each new addon to make sure it stays within reasonable performance specs. Also, I would avoid doing much tweaking directly to P3Dv4 that is not done via the UI controls and vetted addon programs.