I accidentally pressed the mouse buttons that G-ub uses to change mouse DPI, from 4000 to 300. Even though I alt tabbed out and changed it back inside G-Hub, from then on DayZ mouse cursor movement was absolutely screwed and stuck at 300 dpi. Playing with the ingame sliders made no difference. Restarting the game made no difference.

I have accidentally hit the same button pre-1.18 and never had this issue. AFAIK my DayZ is stuck like this forever now in an unplayable state with a barely moving mouse.