re: Electricians please advise! Cox cable melting at tap

One possible cause is that your neutral is grounded in two locations. I believe code requires that your neutral be grounded in only one location (transformer or panel). With two grounds, neutral current may flow through ground path. Not certain of coax grounding, but it has a shield that may be ground path of least resistance. Neutral current would burn up coax in this case. Thus is the reason the ground isolator works.

Just a thought.