This looks very similar to the issue I mentioned to you when we were first working on NWjs, that went away on it's own as the code matured. Not sure that helps any, but it's another datapoint.
The same thing happens to me (see attached images).
In pgAdmin4 v5.1 (Windows 10 enterprise) it's a blank screen. In pgAdmin4 v5.2 (Windows 10 pro) it's looks like it's stuck on the loading screen (but it's already finished loading).
I can reproduce this all day, when I am connecting via an RDP connection (which is how I typically connect). When I go to the same windows machine directly (Windows 10 pro is a physical machine, Windows 10 enterprise is a remote virtual machine) the behavior isn't replicated. The application opens as expected. I suspect it isn't redrawing, or letting the underlying operating system know to invalidate the window when it's done loading in a manner that RDP knows it needs to redraw the window. The local display seems to be smart enough to accommodate for this oversight. I hope that helps.
rik.
[I've updated the bug report, I'm just repeating myself here for those that are curious but don't follow RedMine]