Changed my setup today, previously 24" + 20" monitors, now it's one 30" monitor on which I play four accounts. Mouse still jumps around and one window becomes unresponsive when "focus follows mouse" is enabled, even though there is no secondary display anymore. Or is it due to the higher resolution (2560x1600)?

It works of course when the focus follows mouse option Windows itself offers is enabled. But that requires a reboot and I don't want to have it enabled all the time as it's a bit distracting.