Titanium Backup made Soft Button Bar black

I just wanted to try Titanium Backup on my rooted Nexus 4 (Android 5.0.1), so I installed the free version from the Play Store. Then I chose "Batch" to back up, connected my phone to the charger and turned the display of. After the backup had finished, the first thing I noticed was that my wallpaper was gone and the second thing I noticed was that my soft button bar now is black. Ok, setting the wallpaper again is a matter of seconds. However, I didn't figure out how to restore the soft button bar. Can you help me?

I of course rebooted the device to find out if that helps and switched from Nova Launcher to the stock launcher.

Thank you in advance.