The problem was that I had exceeded the cache limit. Increasing it with APT::Cache-Limit "300000"; in /etc/apt/apt.conf.d/99local cured the problem.
Thanks to Richard Tabor for spotting this. -- To UNSUBSCRIBE, email to debian-bugs-dist-requ...@lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmas...@lists.debian.org