you always find old and weird urls not helping users.
Don't look at that old, wrong and dangerous compatibility list, it's not giving you the answer to your question, it's even not helpful because it doesn't provide correct info.
and Mario kart Wii is working fine, it's not a compatibility list which will reveal your problem.
to verify the IOS slot :
USBLoaderGX > select the game > Option > Game's Option > IOS Slot : verify it's set to "use global". If not, choose "reset" in the previous screen.
then verif your USB Loader GX is properly set to use slot249 or 250.
USBLoaderGX > Settings > Loader's settings > Game's IOS and Loader's IOS : 249 or 250
then go to Settings > Credit > look at the top, it should tell you :
USBLoaderGX r1268 (or 1260 or newer), slot249 or 250, d2x v8 final (or d2x v10 r52), base 56 or 57.
If you have anything else, it's wrong.
One of the Kirby games do have anti piracy check, but not mario kart.
for kirby, disable all patches : be sure you play the game from the same region than your console, disable cheat code, disable "return to" (though it should work with it, you can disabled it to be sure), don't set any video mode or any patches.
If it worked fine few days ago, maybe your device has a problem, your iso is corrupted?
did you add new files to that device ?