Approximately 10 sec after login Ubuntu restarts by it self and goes back to login mode. This happened after latest update from 14.04 to 16.04.

[Evert Håkansson]

A system update from 14.04 to 16.04 was suggested and accepted. After that update the computer goes back to login state after approximately 10 seconds. This behavior is repeated and is impossible to break: So far I haven't found any workaround that gives me access to saved files or other information on the computer.
"Guest login" is possible without the computer restarting but it does not give access saved files on teh administrator account and does nit allow any repair or updating actions.

Please advice what to do to get around this problem and to repair or update the ubuntu 16.04 installation without losing saved files or other information.

Requests:
1) The release of Ubuntu you are using, via 'lsb_release -rd' or System -> About Ubuntu
=> See below

2) The version of the package you are using, via 'apt-cache policy pkgname'
   or by checking in Software Center
=> See below

3) What you expected to happen
=> Normal startup

4) What happened instead
=> The computer starts normally but approx. 10 sec after logging in the screen gets black and the
   computer goes back asking for user name and password.

Other information...

ubuntu 16.04 LTS
Memory: 3.9 GiB
Processor: Intel® Core™ i7 CPU Q 720 @ 1.60GHz × 8
Graphics: NVAS
Operative System Type: 32-bit
Disc: 192.7 GB

/<email address hidden>

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: ubuntu-release-upgrader-core 1:16.04.23
ProcVersionSignature: Ubuntu 4.4.0-105.128-generic 4.4.98
Uname: Linux 4.4.0-105-generic i686
ApportVersion: 2.20.1-0ubuntu2.14
Architecture: i386
CrashDB: ubuntu
CurrentDesktop: Unity
Date: Mon Dec 25 00:54:19 2017
InstallationDate: Installed on 2015-04-05 (994 days ago)
InstallationMedia: Ubuntu 14.04 LTS "Trusty Tahr" - Release i386 (20140417)
JournalErrors:
 Error: command ['journalctl', '-b', '--priority=warning', '--lines=1000'] failed with exit code 1: Hint: You are currently not seeing messages from other users and the system.
       Users in the 'systemd-journal' group can see all messages. Pass -q to
       turn off this notice.
 No journal files were opened due to insufficient permissions.
PackageArchitecture: all
ProcEnviron:
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=sv_SE.UTF-8
 SHELL=/bin/bash
SourcePackage: ubuntu-release-upgrader
Symptom: ubuntu-release-upgrader
UpgradeStatus: No upgrade log present (probably fresh install)
VarLogDistupgradeAptHistorylog:
 Start-Date: 2017-12-17 12:42:31
 End-Date: 2017-12-17 12:42:31
VarLogDistupgradeAptclonesystemstate.tar.gz: Error: command ['pkexec', 'cat', '/var/log/dist-upgrade/apt-clone_system_state.tar.gz'] failed with exit code 127: Error creating textual authentication agent: Error opening current controlling terminal for the process (`/dev/tty'): No such device or address
VarLogDistupgradeAptlog:
 Log time: 2017-12-17 12:40:51.445658
 Starting pkgProblemResolver with broken count: 0
 Starting 2 pkgProblemResolver with broken count: 0
 Done
 Log time: 2017-12-17 12:42:33.869715
VarLogDistupgradeApttermlog: Error: [Errno 13] Åtkomst nekas: '/var/log/dist-upgrade/apt-term.log'

[Evert Håkansson (evert-hakansson)]

Thanks in advance

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.