Ubuntu 14.04, cups

Problem
I spent several hours trying to remove an unnecessary printer in Ubuntu 14.04. Unfortunately, I was unable to pass authentication required for cups administration tasks. It does not matter if WEB UI or Ubuntu System Preferences is used, both are using the same authentication mechanism. I spent a lot of time adding and removing lpadmin users and restarting cups service.
Log file: /var/log/cups/error_log reports constantly the same message:

E [31/Oct/2016:13:35:52 +0100] [Client 8] pam_authenticate() returned 3 (Error in service module)
E [31/Oct/2016:13:42:09 +0100] [Client 8] pam_authenticate() returned 3 (Error in service module)
E [31/Oct/2016:13:43:18 +0100] [Client 17] pam_authenticate() returned 3 (Error in service module)
E [31/Oct/2016:13:54:13 +0100] [Client 17] pam_authenticate() returned 3 (Error in service module)
E [31/Oct/2016:14:01:28 +0100] [Client 17] pam_authenticate() returned 3 (Error in service module)
E [31/Oct/2016:14:04:33 +0100] [Client 17] pam_authenticate() returned 3 (Error in service module)
E [31/Oct/2016:14:07:31 +0100] [Client 19] pam_authenticate() returned 3 (Error in service module)

More talkative was /var/log/syslog and /var/log/auth.log

Oct 31 10:50:18 sb-ThinkPad-W540 kernel: [ 3104.189523] type=1400 audit(1477907418.556:75): apparmor="DENIED" operation="signal" profile="/usr/sbin/cupsd" pid=7913 comm="cupsd" requested_mask="send" denied_mask="send" signal=term peer="unconfined

Solution
The solution is described here. The AppArmor Protection service is badly configured and blocks any attempt for cupds to authenticate lpadmin user.
The workaround is to disable AppArmor temporarily for cupsd

sudo aa-complain cupsd

execute all administrative tasks and when it is done to harden security again

sudo aa-enforce cupsd

I'm not very happy with that but finally I removed disliked printer from my desktop.