Should a N15 I.C. read short-circuit?

[Nick]

The ICP-N15 is a type of self-resetting fuse, much like a polyfuse, but as it's solid-state, it reacts faster and more predictably.

It could be that its out of spec and tripping too early, but frankly I'd look elsewhere first...

I'd start with the ribbon cable plugged it and check each of the various PSU rails to see if any are out of spec when plugged in vs. unplugged...