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Trying to write new Date().toISOString() triggers an error. Apparently the API does not want the time part to be there, even though it's according to ISO spec. Chopping off the time part works.
This seems to be the default now, which presents another pretty fundamental problem: with empty stacks hidden, there is no way to move a card to one of those stacks! Meaning a card can only be moved to a stack that already has at least one card in it...