You’re right, it supports up to 120Hz. But I already set it to 60 also for the 2880×2160 which still works ok. It’s connected via DisplayPort. I wanted to make sure if it’s the monitor or the graphics card that’s causing this. Unfortunately I don’t have a monitor that has a higher native resolution than Full-HD, and don’t know any other way to troubleshoot this issue further.
I also tried adding the resolution using CRU, but that messed up my system completely so I don’t want to use it again.