Cannot unlock session after session is locked for a long time

I have set my laptop to lock the session after several minutes of inactivity. If the session is locked for very few minutes I can see a login window. After entering my password and pressing enter I can get back to work. However if my session has been locked for quite a time (I do not know exactly how long), the login window is missing (I only see the default grayish background with the openSUSE mascot on a plant).

I tried almost every key combination I could think of, but there is no way for me to log in normally. The only option I then have is to press ctrl + alt + backspace twice. This forces a logout, after which I can log back in. Unfortunately if I do this, all apps which were open before locking the session are closed, so I have to start everything anew. What is causing this disappearance of the login window and how can I fix this?

System info

  • Dell XPS 15 L502X
  • openSUSE 13.1
  • KDE 4.11.5
  • Qt 4.8.5

This maybe?

Thanks a lot wolfi32.