Kai, you may want to check U6B pins 14 and 15 on your Sprint layout against the SE PCB scan. Specifically, instead of one trace connected to pin 14 and one connected to pin 15, you connected both traces to pin 15, thus bridging the SCSI ATN and ACK lines. On Apple's PCB:
one trace goes from U6B pin 14, to J5 pin 38. This is the SCSI ACK line.
one trace goes from U6B pin 15, to J5 pin 32. This is the SCSI ATN line.
If you look at the 5380 datasheet (page 3, lower right, "PINOUT") and a SCSI pinout, it should help clear things up. I've attached an image highlighting these pad