Well, for sure it's strange.
I suppose the units are currently both in the same LAN behind the router.
Put static IPs for both DNS units.
For testing purposes put a gigabit switch in between (replace the router). See if it works. If it works with the plain switch then the culprit is the router. I'm not familiar with DIR-615 so I can't help here very much.
Maybe the gigabit switch integrated in the router it's a little picky. Try changing the patch cords with brand new cat.6.
This is all I can think of for now.