"Divide and conquer". Break things in "two" and test each half, keep breaking it down and you'll find the fault quickly. Or be methodical, tear it all apart and start with testing each bit--but that's just slower, not necessarily any better, although verifying each piece is a good idea.
If you don't have a meter, buy one. Or grab an automobile high-beam headlight (used to be you could get one for free in the trash, a dual-beam is thrown out when the low beam blows usually, leaving the high beam working) since that will test voltage and amperage at the same time.
Hook the array up to the lamp or meter, confirm what is coming out for voltage and amperage UNDER LOAD. (The headlight provides that and does that, the meter alone wouldn't.) If you've got a corroded junction or thinned wires, they may put out full voltage but under load that may drop to a
single digit, and that's all it takes to kill things.
If the array tests out good then there is a problem between it and the controller, or in the controller. (Less likely since this is the 2nd one.) If the array does not test out good, then you break out each
single panel and test them to find out which is the problem. Same way, load test each panel and check all junction points. If you find any plain copper wiring (un-tinned) that has started to go green or black from
corrosion? Condemn and replace with tinned wiring.