Touchscreen only works when touching the case

I bought an used SP5 and replaced the screen. All went perfect and no issue seemed to be present. But after using it more, I found a problem with the touch recognition. I can only use my finger to interact while touching the Metall case of the SP5, when laying it on the table (or any other surface) it won't recognize the touch input. I tried every other troubleshooting tip (reinstalling drivers, firmware, resting or setting the calibration etc).

I do have a screen protector on the screen. Might this be the problem? Or do I have to open the device again? Maybe there's some current acting as phantom touch on the whole screen? Magnetic interference? Surface pen works flawless.