I ran into a problem very similar to the one described in Debian bug
868568: deleting a user with a UID < 100000 failed because of a process
that was running as a user with a UID >= 100000. It turned out to be
because the larger user ID was recorded in /etc/subuid as a subordinate
user-ID for the lower-numbered user. Blanking out /etc/subuid and
/etc/subgid caused deluser to behave as normal.
According to login.defs(5), the default range of subuids starts at 100000.
If you're using UIDs over 100000 for normal users then you probably want
to change that (or find a way to disable subordinate user-IDs entirely).
--
Ben Harris, University of Cambridge Information Services.
