I'm having the exact same problem and believe others are too.
Wii: 4.0 USA
USB Loader GX: Forwarder into the Rev 622 version
cIOS35 rev 13 (not the B version).
I can play any game after a cold boot. Then, I exit the game into the Wii System Menu, relaunch USB Loader GX and any game I select will result in a black screen even if I select the same game I was just playing.
EDIT: I'm not sure what the source of the problem was so I decided to try different versions of USB Loader GX. Here are my results:
Channel version Rev 516: Same problem
Forwader + Rev 409: Same problem except I do not get a black screen. Instead, USB Loader GX goes dim and the music continues to play, however it's essentially frozen.
Don't mean to hijack your thread Cru, but you're the first post I've seen that sounds like my problem too. I think the problem might be cIOS.