Galaxy S5 / Lollipop: video player not showing video, only audio

I have a strange problem with my Samsung Galaxy S5 running Android Lollipop 5.0 with all available OTA updates up to today.

I wanted to remote control a specific app from my Mac and tried out a few 'remote control' apps from the App store for the purpose. Some of them required access to the screen (recording / screenshots) upon first start. Unfortunately, none of them worked properly, so I uninstalled all of them again.

But since then (I think this is related but I don't really know), I cannot play my recorded videos any more using the built in "Videos" app. I only get a black screen with a large grey "screen mirroring" icon and I can hear the sound. It seems something activated screen mirroring but I cannot deactivate it in the app or in the settings, and there are no mirroring devices attached either.

Youtube and other video apps seem to work.

What went wrong here? How can I completely disable this feature and get my video screen back on my smartphone? "Screen mirroring" in the Android settings is disabled and was never enabled (by me).