I found this software a couple of days ago. Not sure if it will really help with your first problem 😆 because I'm still figuring it out myself. But looks promising. https://github.com/wwmm/easyeffects

Solved: Ok, it seems to have been a software problem. That had to do with the sync. I have installed ubuntu on the laptop for now and everything works again. Some update seems to have shot something. I am now using a

refresh rate auto switching enabled

not that I am aware of. How could I find this out?

Edit: I want to add that this problem occurred from one day to the next. I'm using Jellyfin as playback, but this effect even occurs when moving windows over the screen

Based on the screenshot I assume that you used youtube premium lite (what ever this was...) on your smartphone. Therefore, here the obligatory recommendation for NewPipe. Quasi Youtube Premium as apk. It has all the features you need:

• no ads
• you can close the app and your videos keep running in the background
• you can turn off the screen and your videos will continue to run in the background

There is also the fork NewPipe x SponsorBlock (link to repository) which integrates SponsorBlock into the app. (duh...) This will also skip the creator's ad blocks in the video.