I am also suffering this same issue. Roon was working fine on Thursday, running from my main screen. Came to fire it up this morning, and it opens on my secondary screen and hangs immediately after opening the window.
If I open the Room app on my iphone, then it connects to my Rock server and plays just fine, so it’s just the Windows client application appears to be at fault.
[edit] It’s definitely due to the secondary screen - if I switch off the secondary monitor, then attempt to launch the Roon application, then it launches fine.