Same here.

which login manager do you use? on openSUSE we pass -DKDE4_COMMON_PAM_SERVICE=xdm, so xdm is required (also is a packaging bug that xdm is not hard-dep for plasma5-workspace package)

Yast's sysconfig editor shows, that DISPLAYMANAGER="kdm"

I tried xdm. No result.

Please check if the file /etc/pam.d/kde exists on your system with the following content:

#%PAM-1.0
auth        required    pam_unix.so nullok
account     include     system-auth
password    include     system-auth
session     include     system-auth

If it doesn't, please test + report whether creating it fixes the problem for you.
Instead of locking yourself out of your session, you can also verify this more easily using `kcheckpass` on the commandline.

There was no such file. I created it, still could not unlock. kcheckpass does not exist in my system nor I could find it in zypper. I am using opensuse rpms.

the path in openSUSE packages is /usr/lib(64)/libexec/kcheckpass. and /etc/pam.d/kde won't help, as we use /etc/pam.d/xdm, as mentioned in comment 2.
but please report back after manually checking with kcheckpass

 /usr/lib64/libexec/kcheckpass 
Password: 
Information: Permissions on the password database may be too restrictive.
Authentication failure

that comes from PAM; can you paste output of:
grep PERMISSION_SECURITY /etc/sysconfig/security

grep PERMISSION_SECURITY /etc/sysconfig/security
PERMISSION_SECURITY="easy local"
# PERMISSION_SECURITY. If PERMISSION_SECURITY contains 'secure' or

The problem seems to be with the permissions on /etc/shadow

chmod o+r /etc/shadow fixes the above issue, however this file should not be world readable

Alternatively, chmod +s /usr/lib64/libexec/kcheckpass temporarily fixes it until plasma5-workspace is reinstalled.

sorry, seams to be a distro issue. Especially comment #12 shows this:
install(CODE "
    set(KCP_PATH \"\$ENV{DESTDIR}${LIBEXEC_INSTALL_DIR}/kcheckpass\")
    execute_process(COMMAND sh -c \"chown root '\${KCP_PATH}' && chmod +s '\${KCP_PATH}'\")
")

kcheckpass is still chown to root and made suid. If distros don't do that they should ensure that kcheckpass can interact with PAM (that works fine for example on Debian).

I got this as well after a distribution upgrade (openSUSE 13.2 -> openSUSE Tumbleweed), also switching from KDE4 to Plasma 5. /usr/lib64/libexec/kcheckpass reported the same access denied error, and chmod +s /usr/lib64/libexec/kcheckpass indeed fixed the problem for now. Although this is marked as resolved, the issue happened today with the latest Tumbleweed repository, which tells me that something nasty is still up.

(In reply to Mircea Kitsune from comment #14)
> I got this as well after a distribution upgrade (openSUSE 13.2 -> openSUSE
> Tumbleweed), also switching from KDE4 to Plasma 5.
> /usr/lib64/libexec/kcheckpass reported the same access denied error, and
> chmod +s /usr/lib64/libexec/kcheckpass indeed fixed the problem for now.
> Although this is marked as resolved, the issue happened today with the
> latest Tumbleweed repository, which tells me that something nasty is still
> up.

This happens to me after updating (zypper up) after which /etc/shadow regresses  to read-only. Really nasty experience.  The workaround is to switch to text console with Shift Strg F1, login, fix the password shadow file as described in comment #11 and then switch back to KDE session with Strg F7 (sometimes F6).

(In reply to Mircea Kitsune from comment #14)
> Although this is marked as resolved, the issue happened today with the
> latest Tumbleweed repository, which tells me that something nasty is still
> up.

Please report this to the openSUSE developers to fix this. It's nothing we at KDE can do about. We only provide software which gets distributed by e.g. openSUSE. Such integration problems need to be addressed by the distribution.

(In reply to Martin Gräßlin from comment #16)
> (In reply to Mircea Kitsune from comment #14)
> > Although this is marked as resolved, the issue happened today with the
> > latest Tumbleweed repository, which tells me that something nasty is still
> > up.
> 
> Please report this to the openSUSE developers to fix this. It's nothing we
> at KDE can do about. We only provide software which gets distributed by e.g.
> openSUSE. Such integration problems need to be addressed by the distribution.

It is known to openSUSE:
https://bugzilla.opensuse.org/show_bug.cgi?id=931296

And it's only a problem for systems that have been upgraded since years (before 12.3 which came out March 2013), because the PAM config is not modified automatically. On fresh installations (or systems originally installed with openSUSE 12.3 or later) this should be ok.

On a side note: The permanent and correct solution is seemingly to update your PAM configuration. This can be done with the following commands, which I've ran and they seem to work okay for me:

pam-config -d --unix2
pam-config -a --unix