Use 120.8Hz custom refresh rate. Both Windows and the monitor’s OSD will show 121Hz. Actual 121.0Hz will cause the error without fail, it must be a tiny bit below. I guess there is something weird going on with the tolerances in the monitor – I’ve noticed things like mains current spikes contribute to the problem.
This may apply more to HDMI 1440×1080 or 1280×720 than to DisplayPort 1080p.
My exact settings for 1440x1080x120Hz over HDMI are as follows: