Today I got the error message a user 'exists, but is not a system user'. I found an entry in the debian mailing list mentioning this problem for the user colord. For me, a couple of other packages and users are affected.
Most important one is the user 'messagbus', as a consequence dbus and related packages fail to configure.
https://www.mail-archive.com/debian-bugs-dist@lists.debian.org/msg2023220.html (https://www.mail-archive.com/debian-bugs-dist@lists.debian.org/msg2023220.html)
FYI: I was bitten by this bug this morning during a dist-upgrade, but I updated and retried it this afternoon, and it went without problems. Some fix was probably uploaded.
"Some fix" was package adduser 3.143 -> 3.144 ;)