An open battery in a string- risks

At this point, it is a failed battery. And needs to be replaced. Having ~400-768 VDC across a failed battery is just asking for risk of fire (strike/sustain an arc across the failed internal connection).

Longer term, have you been monitoring the battery bank (volt meter across each cell/battery to make sure all of the neighboring batteries are working correctly? If flooded cell batteries, have you been checking the specific gravity of the cells ~once per month (and electrolyte levels)?

-Bill