If you are using iOS then grab the free version of 8Player and that is able to find SMB shares and UPnP servers on your network - see if that sees your Minimserver?
There’s nothing changed in either the Varese or Mosaic ACTUS as far as UPnP discovery is concerned so my suspicion would be that there is (as has been suggested by @Anupc earlier) an issue with Multicast traffic being broadcast around your network.
I HAVE had issues with this happening with NetGear Orbi kit before but that was a very long time (several years) ago and as far as I am aware those issues should have been resolved so I’m somewhat surprised that swapping to newer kit has introduced this issue … unfortunately it’s almost impossible to remotely diagnose these consumer level mesh systems as a full setup and generally my suggestion would be to break the system down into its simplest possible setup and then add to it.
So, although I realise it is a total pain to do, I’d pull the router, power down every WiFi extender, set the router up with just the Varese, Minimserver and Audirvana PC connected to it by WIRED connections and confirm that UPnP works there, then from there add back in devices until things stop working and that will usually give you a way to finding out what is breaking stuff.
Ultimately, as @Anupc has already mentioned, consumer grade mesh networking kit really isn’t at a point yet where it is reliable enough to confidently use as a solution - which is why we always recommend against these solutions and why as a first port of call when diagnosing issues I’ll ask exactly what is between the dCS kit and the router and ask for the kit to be directly connected back to the router and any WiFi Extenders turned off. The general level of support that you’ll get from the manufacturers on this consumer grade kit will normally be essentially a couple of questions about how it is set up and then a not very helpful “in that case box it back up and return it for a refund” but then again WiFi is totally at the mercy of the local environment and there just isn’t the meat on the bones of those products (nor the configuration ability) to allow for much in the way of faultfinding and resolution… 