If I end a video (regardless of the resolution) and then immediately switch off the drone, this video is defective - it cannot be played (but has a file size corresponding to the recorded time).
If I wait 1 minute after finishing the video and then turn off the drone or finish the video and take a photo and then turn off the drone, there are no problems.

Why is that? Can i adjust something there (I deactivated the cache for the mobile phone)?

I am using this SD card:
SanDisk Extreme microSDXC 64GB 160MB / s A2 C10 V30 UHS-I U3
It should even be more than fast enough for 4k60FPS (is it in a GoPro anyway ...)

May I ask which drone you are using to record 4k @ 60fps ?

I wrote that the SD card is fast enough to also record in 4k60 ... not that the Mini2 can - so the problem is not with the SD card ...

Does it also occur if you don't use the SD-card ?

I do not understand the question ... how should I record a video if there is no SD card inserted?  I have this behavior with different SD cards, all of which I use successfully in a GoPro Hero 8 and Hero9 (even with a 5k30).  It always happens when I switch off the Mini2 immediately after stopping the recording.  It looks like the drone needs a little more time to properly save the video ...

If you turn off the drone before stopping the recording the file will be defective, it needs some seconds to end the file properly, so just stop recording before turning the drone off. I usually stop recording just before landing.

The drone will auto repair the defective file if you reinsert the SD with the damaged file and wait a minute or so, but it doesn't always work.

It happened to me once, and since then I always stop the video recorduing just before landing and never happened again.

Thanks for the information!
Of course, I finish the video and only then switch off the drone - but apparently too quickly immediately after the end of the video.
I will try next time that I switch the drone on again with the SD card inserted to repair a possibly defective video.  
It might be a consideration for the next update - if the video is still recording and the drone is switched off, the video should first be properly ended and saved before the drone actually switches off.

Hi, ozoffi. Thanks for reaching out! Commonly, there would be more pixels included in a video compared with the photo with the same device, which means that there might be more time required to fully write them into the storage medium, and powering off the aircraft would end the process.
In order to ensure completeness, it is recommended not to remove the MicroSD card or power off the aircraft immediately after the video ended.