I have never managed to get the Web Display to work on Safari, Chrome, Chrome Edge.
All I see is a dark gray with word “Roon” in the middle after copy and paste the LINK to the browser address bar.
I have restarted the Roon Server several times on the Mac Mini, and I have also tried the Roon Server on a Windows 10, on a QNAP, none works.
As I already told you in the past, this is the default page. Everything looks as expected so far. Your screenshot of the Displays section shows that your browser is connected to Roon and ready to be used.
PS: Connecting the display to a zone with empty queue may remove the question mark.