If you were able to try 3.16-1~exp1[0] from snapshot.debian.org that might help rule that out.
I downloaded linux-image-3.16-trunk-kirkwood_3.16-1~exp1_armel.deb from your link and flashed it into the NAS. I have not done a deeper analysis, but it looks like the problem is gone.
For now, I will revert back to the known-working 3.14-2 (3.14.15-2) anyway, but if you want me to test another binary image, I could do that if given a week or two.
Hope this helps.