I think I have encountered a bug:
I created a new Subtracks instrument and loaded samples into the first seven sample slots. I saved this as a preset but when I tried to load this preset I got a “Could not load preset” error message. I then created a new Subtracks instrument from scratch again, loaded samples into the first two sample slots and saved this as a second preset - this one also produced the same error message when attempting to load it into another pattern.
After reading the manual and trying to figure out what I could be doing wrong I had a thought - so i tried the same procedure again, but this time I loaded samples (actually the same one) into all eight sample slots. Now, after this preset was saved I could successfully load it onto another track.
My conclusion: saving a preset from a Subtracks instrument that does not have all its sample slots occupied can/will produce an unloadable preset.
Maybe someone could try to reproduce this?