Ok, i might be a step further now.. Did not know you had to actually restart the PC after adding a custom resolution. Now 2560 x 1440 is showing up in Windows and i can select it, however when i select it, there are some weird distortion effects going on, the whole image on the screen is moving around until it automatically changes back to 1920x1080 after a few seconds. I tried this now with different settings on my custom 2560 x 1440 resolution, such as 30 hz, 35 hz, 29 hz, "timing standard" set to GTF and CVT-RB (no idea what all this stuff means).
It's always the same result though. I switch the resolution to 2560 x 1440 and it appears to be the right resolution, judging by the font sizes and sharpness and so on, only that, as described, the image on the screen is jumping all over the place and there's these green distortion effects showing up, and then it changes back. Isn't this a sign of too much hz selected? Anyway, i'd be very glad if someone could help me with this.
This was all done through the HMDI port btw.