[kaptain]

Thanks for your update. Glad to see you are making good progress to the point near final resolution.
If that error message comes up again. I would recommend to take a measurement on the battery voltage. Measure each battery if they are connected separately to each Outback.
I think it is also possible that one or more battery group being too weak (below 12v) resulting what you saw, and maybe it’s time to replace that weak battery, but I think this is low probability because in such a case, you should get some message showing on the Outback inverter.