Paul L was kind enough to take a look at the problem this morning. He remotely forced faulty battery to charge enough to rise above the minimum level to operate, and since then it's been behaving normally, allowing the secondary battery to behave normally, and is charging from the solar (when we actually had some before the thunderstorm rolled in, and before we started the laundry!). It remains to be seen whether the battery will continue to behave as it should, and I'm monitoring it closely.
Paul doesn't think it was the SOC issue.
We have a 5.0 HY Gen 2, and are on inverter s/w 909 and battery f/w 3009.
I'm reluctant to touch the cabling, eg the BMS, because I don't know what I am doing.