I get the red box 'Reload Simbrief' - the Fenix loads the current flightplan from Simbrief correctly ok
These two events are completely unrelated: nothing in GSX has any effect on how the Fenix would load (or not) a Simbrief flight plan. Yes, of course, normally they final effect should be the same, but not because GSX and the Fenix are connected to each other, simply because, if they are reading, each one separately, the same flight plan, the data should be the same.
Simbrief works just fine here. When the "Reload Simbrief" button remains red after you click it, it doesn't "just" stay red: it will print two messages: one on top of the screen, indicating the problem, and some text under the button, saying something about the problem as well.
What does it say, after pressing the Simbrief button?
Also, note the button won't immediately refresh and turn green on the menu, if the flightplan has been loaded, you'll see it from the top message which will say something about the payload and passengers, but the menu will go green for the next time you open it.