Because I had a problem with an anti cheat tool for CS:go I updated my nvidia driver to the newest from 347.25 to 355.60.
After the update I wasn't able to switch to 144 hz anymore, the driver (even manual addition of the native resolution with 144 hz) didnt allow me to let it run with 144 hz neither any resolution! Also the OSD wasnt able to force it to 144 hz. I also downloaded the display driver and the problem was still there.
After a rollback to 347.25 everything went back to normal.
