can not open timekpr gui

Can you give the output of this command:

cat /etc/passwd | cut -d : -f 1-3

P.S. The development of this project is practically dead. Please prefer Gnome nanny or some other software, e.g. Mkahawa Internet Cafe management software: https://www.unwiretechnologies.net/downloads.php

Here it is:

gb4@dell530n:~$ cat /etc/passwd | cut -d : -f 1-3
root:x:0
daemon:x:1
bin:x:2
sys:x:3
sync:x:4
games:x:5
man:x:6
lp:x:7
mail:x:8
news:x:9
uucp:x:10
proxy:x:13
www-data:x:33
backup:x:34
list:x:38
irc:x:39
gnats:x:41
nobody:x:65534
libuuid:x:100
syslog:x:101
messagebus:x:102
avahi-autoipd:x:103
avahi:x:104
couchdb:x:105
speech-dispatcher:x:106
usbmux:x:107
haldaemon:x:108
kernoops:x:109
pulse:x:110
rtkit:x:111
saned:x:112
hplip:x:113
gdm:x:114
gb4:x:1000
laure:x:1001
antoine:x:1002
maxime:x:1003
claire:x:1004
invite:x:1005
statd:x:115

Note: admin is gb4 (me)
By the way, I have de-installed timekpr of this machine but I guess it does not affect passwd file.
For what concerne Nanny: I already tried it, but it did not limit the time at all (see https://bugzilla.gnome.org/show_bug.cgi?id=644323 ).
What'the relation ship between Mkahawa Internet Cafe and parental control???

Can you also give the output of this command:
sudo cat /etc/shadow | cut -d: -f1

The error seems to say that there is a user called "dhcp", we'll see if this assumption is true.

> What'the relation ship between Mkahawa Internet Cafe and parental control???
Well none, but timekpr was used as parental control and as a time limitation software (similar to how internet cafes work). So I gave you the choice, if you're interested in parental control software, gnome nanny is the way to go. :)

This question was expired because it remained in the 'Needs information' state without activity for the last 15 days.

Here is teh output of the required command:

gb4@dell530n:~$ sudo cat /etc/shadow | cut -d: -f1
[sudo] password for gb4:
root
daemon
bin
sys
sync
games
man
lp
mail
news
uucp
proxy
www-data
backup
list
irc
gnats
nobody
libuuid
dhcp
syslog
klog
hplip
avahi-autoipd
gdm
pulse
messagebus
avahi
polkituser
haldaemon
gb4
laure
antoine
maxime
claire
Debian-exim
invite
usbmux
couchdb
speech-dispatcher
kernoops
rtkit
saned
landscape
statd
freeswitch
mysql
tomcat6
red5
postgres
gb4@dell530n:~$

It does show an 'user' dhcp. I don't know where it comes from. May be a fossil from a previous installation of a BigBlueButton server (the most complicated networking stuff I tried on this PC, but which I uninstalled since that time: I had too many problem with it).
I also have no idea if this dhcp user is still used by anything or not. How can I know?

No idea. :) All I know is that if you remove it you will fix the problem.

For example, in my case, I have identical users in /etc/passwd and /etc/shadow files

Actually in your case, you have a lot of differences between /etc/passwd and /etc/shadow:

$ diff -ur test_passwd.txt test_shadow.txt
--- test_passwd.txt 2011-10-04 22:56:29.000000000 +0200
+++ test_shadow.txt 2011-10-04 22:56:52.000000000 +0200
@@ -17,24 +17,34 @@
 gnats
 nobody
 libuuid
+dhcp
 syslog
-messagebus
+klog
+hplip
 avahi-autoipd
+gdm
+pulse
+messagebus
 avahi
-couchdb
-speech-dispatcher
-usbmux
+polkituser
 haldaemon
-kernoops
-pulse
-rtkit
-saned
-hplip
-gdm
 gb4
 laure
 antoine
 maxime
 claire
+Debian-exim
 invite
+usbmux
+couchdb
+speech-dispatcher
+kernoops
+rtkit
+saned
+landscape
 statd
+freeswitch
+mysql
+tomcat6
+red5
+postgres

"-" means: exists only in passwd
"+" means: exists only in shadow

I don't know what you should do, but you have a lot of differences between passwd and shadow. I don't know if users can exist only in shadow and not in passwd (and vice versa). :)

> "-" means: exists only in passwd
> "+" means: exists only in shadow

I made a mistake, I did not sort the entries. I think the only problem is dhcp in shadow.
(I'm really sorry for multi-commenting!)

Problematic user entries in shadow (exist only in shadow file):

Debian-exim
dhcp
freeswitch
klog
landscape
mysql
polkituser
postgres
red5
tomcat6