@fillilutten
Whats the GC game path and used format? ISO is recommended. Check
here and best use GC Backup Manager to verify the image is not corrupted and will be copied to correct game path automatically, As USBLGX needs a specific game path with game ID, which Nintendont doesnt require.
Are you launching USB Laoder GX through the Forwarder Channel or HBC?
//EDIT: Nintendont version is latest v6.500 ?
And check all Nintendont settings inside USBLGX, e.g. if forcing some video mode is turned on or patch is activated.