c'est aussi mon opinion
Did you try a simple app like ain64_din128_dout128_v2c.zip ? when you connect SI to ground and release, it sends all the 128 midi note on/off messages
after that, connect the DIN to J9, and test each Input pin of serial registers (don't forget pull up 10K resistors).
when connected to ground it sends note On, note off when released. more over, note start from 0 and goes to 128 (if 4 DINx4 chained), indicating you which input of the 74HC165 could be dead (i had the problem 2 years ago)