At the risk of redirectly the discussion back to the hot cable, the location of the heat is the location of the problem. For example, if one side of the fuse block is hotter than the other, look on the hot side. I suspect that the crimp on the cable terminal has either some corrosion
in it, or it was never quite crimped correctly, and there is resistance.
It's doubtful that the problem is under the insulation
of the wire, unless the boat environment
is really wet. It's either the terminal crimping or it's the security
of the terminal on the fuse block.
Since I didn't have one of the big "bolt cutter" style tools on board, I used the old bolt-together Nicopress tool for most of my big battery cables
with very good results.
See WEST MARINE Hand Swaging Tools | West Marine
I know this isn't entirely to code but for all terminals that I used, I could find a die that put great pressure on the wire. I also used the Ancor adhesive-lined shrink tubing for strain relief and insulation