In a production area less than a year old, an 800A feeder supply breaker tripped. Before attempting a reset, you performed insulation resistance tests on the conductors. Resistance was low enough to indicate the insulation had melted.
Obviously, the breaker mechanism wasn't frozen or the breaker wouldn't have opened at all. Your electrical testing firm ran a complete diagnostic on the breaker, and it performed flawlessly. A review of the power monitoring logs showed no excessive current until just before the breaker tripped. This indicates that the melting caused the excess current, not the other way around.
What should you examine next?
You might want to verify a few other things, such as whether excessive harmonics were on that circuit or a transient event occurred. However, given the age of the installation, the most likely cause is damage to the conductor insulation during installation. If possible, find out which electricians installed that particular circuit and ask them about the cable pull. Was there any particular problem with it? Look at any other circuits they installed because, regardless of problems, a damaged cable should have been discovered and replaced.
If you have a good insulation-resistance testing program, then you have baseline data for all installed cables. Compare the readings for similar cables to see where any other "Easter eggs" are waiting. Also look at the trending. Where you spot anomalies, it's more economical to schedule a cable replacement than to run until failure.