hey! it happened to me, I did some comprobations that might be useful to you:
The config file needs to be working properly in the editor, so you need to be seeing the squares representing the monitors in a proper manner. If you don’t see that, you are probably doing something wrong with the config with that black screens as a result. The debug is not clear enough if you have an issue with the config.
Another possible cause is the need for selecting in the display cluster settings (within the editor, the actor blueprint that sets up cameras), an actual monitor that works in your computer… somehow, the examples are a bit messy with that, so you might be selecting a wrong output as the main display, so check that out.
I also succeeded better using 4.20 (4.21 was somewhat more glitchy)
if you happen to know how to render more than one display in a single computer please let me know! i have a question about it here