Hey,
Sure so these are the displays I'm using:
https://www.amazon.de/ST7789-Drive-Display-Module-Vollfarb-LCD-Anzeigemodul/dp/B07QGPKWJ2
I'm following the connection layout given by this video at this timestamp:
https://youtu.be/OW-Fcv26p9o?t=89
They seem to use the same display.
One thing I just noticed: When I put my PC into sleep mode but leave the arduino on the USB port, the display shows everything fine. So it seems there is something going on with the output voltage of the USB port? But it also doesn't seem to be specific to this USB port since I tried it on multiple PCs and I had the same issue everywhere. I also tried different USB cables so this is also not the problem.