I'm just fishing here, as I do not have any experience with Dante ... but you mention using three zones, and this is a pretty generic error message. This can happen when you have playback (or music cued sometimes) in one zone and you shift to another zone.
You can test this by looking at the overview under playing now and see what is marked "Current". Manually click on the stop button (or even clear playing now list in the zone).
If this has something to do with it, you can make zone switch rules to automatically stop playback in one zone, when another is playing.
Of course this may have nothing to do with your issue! For all 6 channels to playback at once they normally have to be in one zone (unless they are linked, and that would introduce other latency issues to compensate for I think. But as I said I have no idea how to use a vritual sound card in this way -- at least it serves to bump your post
good luck
PS - 48K sample rate is a pretty strange general setting -- this might have something to do with it too?