Cannot log in after upgrade: xcb plugin missing

I have Kubuntu 15.10 with kubuntu-ppa/backports and kubuntu-ppa/ppa enabled. Yesterday some updates came (5.5.3 I guess). Now I cannot log in:

.xsession-errors

This application failed to start because it could not find or load the Qt platform plugin "xcb".

Available platform plugins are: linuxfb, minimal, minimalegl, offscreen.

Reinstalling the application may fix this problem.
startkde: Starting up...
dbus-update-activation-environment: warning: error sending to systemd: org.freedesktop.DBus.Error.Spawn.ChildExited: Process org.freedesktop.systemd1 exited with status 1
kdeinit5: preparing to launch 'libkdeinit5_klauncher'
kdeinit5: Launched KLauncher, pid = 2221, result = 0
This application failed to start because it could not find or load the Qt platform plugin "xcb".

Available platform plugins are: linuxfb, minimal, minimalegl, offscreen.

Reinstalling the application may fix this problem.
kdeinit5: Communication error with launcher. Exiting!
kdeinit5_wrapper: Warning: connect(/run/user/1000/kdeinit5__0) failed: : Connection refused
This application failed to start because it could not find or load the Qt platform plugin "xcb".

Available platform plugins are: linuxfb, minimal, minimalegl, offscreen.

Reinstalling the application may fix this problem.
Aborted
/usr/bin/startkde: 1: /usr/bin/startkde: kreadconfig5: not found
startkde: Shutting down...
kdeinit5_wrapper: Warning: connect(/run/user/1000/kdeinit5__0) failed: : Connection refused
Error: Can not contact kdeinit5!
startkde: Done.

I have installed manually libkf5config-bin, and kreadconfig5 is now available, but the message "This application failed to start because it could not find or load the Qt platform plugin "xcb"." still persists not allowing me to log in.

Should I wait for the next updates? Or I can fix this now?