We sorted it out - thanks for the great cooperation, Mads!
It was because the Remote Latency setting was set to 0.1 seconds, with no appearent reason - that’s way too few, and we’ll make sure that it can’t be set that low in upcoming versions. Remember it takes time for the headphone mix to travel to the Performer, and it takes time from the Performer back to Cubase, both with latency and the internet in between which is often unpredictable. So you should set it to 1.0 seconds or so to be on the safe side.