Hi Guys, just wanted to share my feedback here.
I had this same issue, Related Products just wont work until I typed in the full category name ( just pick any name from your existing categories ) like Music, Speakers, etc, whatever you got!
Type in the full category name in the related products search field and it should automatically load all products available in that category for you to select. Just make sure the full name word is typed and no spelling mistakes.
I spent about 2 days banging my head here and the solution was right in front of me without any coding / debugging required.
I hope this helps!
Worked for me.
Thanks