user "daisy" can log in but user "karel" gets a blank X-screen

[Karel]

When logging in as "karel" I get a blank X screen, which is perfectly functional as such - from a terminal I can launch "firefox --display :0" and so forth - but user Karel does not get a window manager active. For newly created user "daisy" everything works as expected. Not sure this is a bug, though, the trouble started after some twiddling with adding and removing packages.

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: lightdm 1.10.5-0ubuntu1
ProcVersionSignature: Ubuntu 3.13.0-51.84-generic 3.13.11-ckt18
Uname: Linux 3.13.0-51-generic x86_64
ApportVersion: 2.14.1-0ubuntu3.10
Architecture: amd64
Date: Sun May 3 20:31:50 2015
InstallationDate: Installed on 2014-04-29 (369 days ago)
InstallationMedia: Ubuntu 13.04 "Raring Ringtail" - Release amd64 (20130424)
SourcePackage: lightdm
UpgradeStatus: Upgraded to trusty on 2014-05-01 (367 days ago)

[Robert Ancell (robert-ancell)]

Sounds like a broken session for some reason - is this still occurring?

[Karel (karlchen9)]

The behaviour has slightly changed but it seems clear to me that the same issue is still at work.
At the prahical login, there is now a list of accounts to choose from, it does include "daisy" but not "karel". There is however a "not listed" to click, and that allows me to enter "karel" as login, after password validation everything works as expected.

My understanding was and is that the non-working user "karel", though mentioned in the basic /etc/passwd and /etc/shadow, is missing from some other list of users, proper to lightdm. I have however failed to find such a file, perhaps the account names are encrypted into it?