Erwin, your setup looks pretty reasonable, I can’t immediately tell if there’s anything wrong with it as such.
That said, you should probably check and turn OFF (as in un-check) a couple of settings on both your Router and the Extender (if you don’t already have the User Manuals, get them online; R6400v2, EX7000)
On the Router, be sure to turn OFF UPnP (see Page 54), and turn OFF DLNA (see Pages 94 and 96), and turn OFF IGMP (see Page 107)
On the Extender, turn Off DLNA (see page 56).
By the way, I assume the Extender is connected via an Ethernet cable to the Router, and not over WiFi? If it’s interconnected over WiFi, like in a WiFi Mesh, it would be a bad idea to separate the NAS and Bartok, and you might want to put them together on the same switch instead.