Galaxy S5 / Lollipop: video player not showing video, only audio
Solved. It was actually this app: https://play.google.com/store/apps/details?id=com.teamviewer.quicksupport.addon.samsung

After deinstalling this app and restarting my phone the Android video player app works again. Just deinstalling Teamviewer Quicksupport itself neither deleted this add-on nor helped with my problem.

EDIT: Also, deinstalling this app removed very strange lags and pauses at certain display animations (fade effects, e.g. of the lock screen camera button and in apps) that I experienced while I had this app installed.