We really should have known from the beginning if this is an (intentionally) grounded system or not. In your last post you finally mentioned Outback, whose systems AFAIK are all intentionally negatively grounded, so I'll proceed on that assumption.
Since the negative-ground connection is at the inverter, any string that does not have it's negative conductor properly connected back to the inverter won't have the negative conductor grounded, and thus no positive to ground-voltage (unless there's also a ground fault on the negative conductor of that string). If you're measuring + to - on the string side of the broken connection, you'll get voltage, which I think is the most likely explanation for your situation. If you measure on the inverter side of the broken connection, you won't get voltage. Since the other strings seem to be grounded via the inverter, the break must be between the combiner busbar and wherever you are measuring. (Golddigger sort of said this.)
Are you measuring at connectors for this string right outside the combiner? If so, did someone install one of them improperly so that they don't make contact when plugged in? When this happens you can often measure the right voltage with your meter when unplugged, but it still doesn't work when plugged.
There are other more far-flung possibilities, involving multiple ground-faults combined with bad connections, but unless you tell us more that doesn't seem worth discussing.
It may also be worth noting that in grounded systems you cannot properly diagnose many string problems unless all negative and positive conductors are disconnected from the combiner busbars. It's usually easy to do the positives with fuses, but the negatives require more planning ahead, such as installing extra connectors outside the combiner.