I don't have an Ultimate64 but I can give some insight. Did you mount a blank .D64 image to the ultimate64? If so it would be saved inside the D64 disk image. If the Ultimate64 saves directly to the file system then it might not be named "save1". Reason being that KotSS saves in screen code names not PETSCII. This is the chart for screen codes: https://www.pagetable.com/c64ref/charset/ - in other words your file name would end up being unreadable ASCII control characters if you directly saved it to the USB file system if you didn't use a .D64 for saves, and the file system might not like that. Try saving as just numbers only because those are compatible with the USB file systems, and see if that works.