I got this error quite frequently and USB Loader GX uses IOS 249 have a look if yours does as well.
As USB LOader GX starts up you get a black screen and have a look which IOS it uses if it is 249 then follow the steps in the spoiler, if not simply post back which IOS you are using.
Okay try this - it worked for me with my recent errors and crashing of USB lodaer GX:
Try installing cIOS Installer rev 20b on IOS 249 and then ruunning USB loader. (Network install didn't work for me - you may have to try WAD install - if the file is not on the SD card it will tell you what file you need - just google it and download the wad and place in the sd card root then repeat the rev 20b install choosing WAD install).
If that fails or crashes (or gets a similar code dump) and you have PriiLoader installed make sure you have blocked wii updates.
Next try to perform a wii update from system menu (it will fail) this should then help you with the rev 20b install.
PLEASE BE WARNED COMPLETING A NINTENDO UPDATE CAN BE VERY DAMAGING TO YOUR WII AND MAY BRICK IT - IF THE PROGRESS BAR STARTS GOING UP HOLD THE POWER BUTTON ON YOUR WIIMOTE AND TURN THE CONSOLE OFF.