sddm-helper (login screen) hangs with pam_limits maxlogins

sddm-helper hangs when user tries to login more then maxlogins PAM limit. No mouse/keyboard control possible, only VT change possible.

HOW TO REPRODUCE:

1. Activate PAM maxlogins limit for the user (max 1 session for user basia):

$ echo 'basia - maxlogins 1' | sudo tee /etc/security/limits.d/no-multiple-sessions.conf

2. Start first plasma session from SDDM login screen:

$ loginctl -n 500 session-status 43
[...]
Apr 24 21:12:22 slodki systemd[1]: Started Session 43 of user basia.
Apr 24 21:12:22 slodki sddm-helper[20244]: pam_unix(sddm:session): session opened for user basia by (uid=0)
Apr 24 21:12:22 slodki sddm-helper[20244]: pam_systemd(sddm:session): Cannot create session: Already running in a session
Apr 24 21:12:22 slodki sddm-helper[20244]: pam_kwallet(sddm:session): pam_kwallet: pam_sm_open_session
Apr 24 21:12:22 slodki sddm-helper[20244]: pam_kwallet(sddm:session): pam_kwallet: final socket path: /run/user/1002/kwallet.
Apr 24 21:12:22 slodki sddm-helper[20244]: pam_kwallet5(sddm:session): pam_kwallet5: pam_sm_open_session
Apr 24 21:12:22 slodki sddm-helper[20244]: pam_kwallet5(sddm:session): pam_kwallet5: final socket path: /run/user/1002/kwalle
Apr 24 21:12:22 slodki sddm-helper[20244]: Starting: "/etc/sddm/Xsession" "/usr/bin/startkde"
Apr 24 21:12:22 slodki sddm-helper[20252]: Adding cookie to "/home/basia/.Xauthority"
[...]

3. Switch to new user (new session) and start 2nd user session from SDDM login screen:

$ loginctl -n 500 session-status 46
46 - basia (1002)
           Since: Mon 2017-04-24 21:12:48 CEST; 15min ago
          Leader: 20640 (sddm-helper)
            Seat: seat0; vc9
         Display: :2
         Service: sddm; type x11; class user
         Desktop: KDE
           State: online
            Unit: session-46.scope
                  |-20640 /usr/lib/x86_64-linux-gnu/sddm/sddm-helper --socket /tmp/sddm-auth358e06de-56ea-43cf-a3a2-95f29714e
                  |-20642 /usr/bin/kwalletd --pam-login 15 18 --nofork
                  `-20644 /usr/bin/kwalletd5 --pam-login 15 19

Apr 24 21:12:48 slodki systemd[1]: Started Session 46 of user basia.
Apr 24 21:12:48 slodki sddm-helper[20640]: pam_unix(sddm:session): session opened for user basia by (uid=0)
Apr 24 21:12:48 slodki sddm-helper[20640]: pam_systemd(sddm:session): Cannot create session: Already running in a session
Apr 24 21:12:48 slodki sddm-helper[20640]: pam_kwallet(sddm:session): pam_kwallet: pam_sm_open_session
Apr 24 21:12:48 slodki sddm-helper[20640]: pam_kwallet(sddm:session): pam_kwallet: final socket path: /run/user/1002/kwallet.
Apr 24 21:12:48 slodki sddm-helper[20640]: pam_kwallet5(sddm:session): pam_kwallet5: pam_sm_open_session
Apr 24 21:12:48 slodki sddm-helper[20640]: pam_kwallet5(sddm:session): pam_kwallet5: final socket path: /run/user/1002/kwalle
Apr 24 21:12:48 slodki sddm-helper[20640]: [PAM] openSession: Permission denied

RESULT:
sddm-helper hangs after PAM blocking new user session, logind status of that session is active, must be killed with

$ sudo loginctl terminate-session 46

EXPECTED RESULT:
a) message displayed that session cannot be started due to PAM restrictions
b) logind and X sessions closed
c) seat active session switched to previous one

System: Kubuntu zesty amd64
sddm 0.13.0-1ubuntu5
ibpam-kwallet5 4:5.9.4-0ubuntu1
libpam-modules-bin 1.1.8-3.2ubuntu2
libpam-systemd:amd64 232-21ubuntu3
systemd 232-21ubuntu3