There's an inner groove in the sheave that is supposed to accommodate the wire. It's u-shaped and about 1/4 inch across. The outer grove is for the rope. I would guess that, as the halyard transitioned from rope to wire, it got caught on the edge from the large groove to the inset smaller groove. When we took our sheaves out, they were all cut-up at this transition point. Also, it looked as if the wire had jumped its groove a few times and chewed up the outer groove. Either way, lots of sharp edges.
Another possibility: the clearance between the sheave and the box that holds it is very small. If you have a bulky bit of tape, it could have jammed as it exited the sheave box.
As far as obstructions inside the mast: You have wires for your gear into the masthead. You have 3 cross bars where the shrouds connect to the mast. It's possible that something got caught on a cross guard.
FWIW, we took our sheaves to a machine shop, had them route out the inset groove and went to all-rope halyards for about $90. Plus, that allowed them the put in a new oiled brass bearing; this made for worlds better action and easier pulling.
Our boat: 1981 C&C 32
If I die, my biggest fear is that my wife sells all my toys for the price I told her I bought them for...