Does this have anything to do with this entry in /usr/share/doc/cryptsetup/NEWS.Debian.gz?
The default key size for LUKS was changed from 128 to 256 bits, and default
plain mode changed from aes-cbc-plain to aes-cbc-essiv:sha256.
In case that you use plain mode encryption and don't have set cipher and hash
in /etc/crypttab, you should do so now. The new defaults are not backwards
compatible. See the manpage for crypttab(5) for further information. If your
dm-crypt setup was done by debian-installer, you can ignore that warning.