The error has been changed to a warning in 2c6e6148709fc3accc8c5b442ce021d45f6f3ac5 (-2.10). The underlying issue remains the same - calling umount won't call umount.crypt because there is no mention of crypt in /etc/mtab, so in other words, the crypto and loop devices remain active even after logout. To fully work with such mtab-less (or mtab=ro) systems, the new utab branch from git would be needed.
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