Reformat the SD card to FAT32, use the full/long/overwrite setting instead of the quick one, redownload USB Loader GX and put it on the SD card.I like to take my time on this step to make sure it comes out right. I've found cIOS - Wii Guide by RiiConnect24 explains a good working cIOS configuration. Make sure the cIOSes are in the correct slots and the correct versions.(it sounds like you have already done this so feel free to skip it) I think they have a factory default option. I'm just including what you've done here to try to make the answer as complete as possible in case anyone else needs help in the future.
I think you've already tried some of them so skip suggestions 1 and 2 (and any others you have tried). I might suggest the following trouble shooting steps. It usually seems to relate to the settings in USB Loader GX or a corrupted file on the SD card or usb drive. I've had some similar issues with USB Loader GX in the past.
