Troubleshooting Solar under Load

Ok so I have an issue with one of my strings, in that it decided yesterday at some point to just stop working.

I have:

• Replaced wiring from combiner box to charge controller
• Plugged string into a different charge controller to check if that was faulty
• Checked voltages at every point along the path

It goes something like:

String1 (380v 9a DC) (Multimeter shows full voltage ~373V on breaker)

+ String2 (380v 9a DC) (Multimeter shows full voltage ~373V on breaker)

-> Combiner Box (Registering about 373V DC when I was checking), which should be around 18a under full sun but it’s winter here right now.

Combiner box runs ~40 metres underground 10mm2/8awg double insulated twin conductor (red/black) into the power shed, where it terminates at 1200v/32a DC solar isolator.

1200v Isolator shows 373V on multimeter as well.

-> about 1m to the charge controller in 6mm2/10awg double insulated twin conductor (red/black) solar cable where it measures also about 373v at the terminals of the charge controller.

HOWEVER

As soon as I turn the charge controller ON, so there is load and attempt to charge batteries, it just start fluctuating voltage anything from 70 to 323 that i’ve seen but mostly in the 200’s and produces 0 amps of current.

There is a clone of this array next to it (string 3 + string 4), wired exactly the same into the same charge controller (Victron 450/100 RS with two trackers) and it works fine.

Thinking maybe a panel has died? How do you even test a panel for load?

Like I can disconnect the MC4 connectors and check voltages but they are obviously fine, how do i check for load?