For what it's worth, I bumped into this while upgrading from 1.118 in a pre-release jessie to 1.123 from released jessie. According to http://www.fifi.org/doc/debconf-doc/tutorial.html#AEN198 it's possibly because there are file descriptors left open at the end of the postinst. Calling db_stop explicitly at the end of the postinst file for console-setup fixed this for me.