OK, did a bit of experimenting. Our nodes are running both Max 2013 and Max 2015.
If I send a DR render from a master running 2015 when the nodes have just started, it works fine. DR renders from 2013 master generally work OK, but if I send a render from 2015 master after a 2013 DR render has been running, it gives me the waiting for render start and I get nothing. If I then send a render from 2013 it works fine.
Few screenshots for you to look over attached.
Using four render nodes to start (three running windows 10, 1 running windows 7)
The first batch was captured when I sent a render from a 2015 machine.
No.2 render from a 2013 machine
I then turned on another DR node
No.3 render from a 2015 machine
last render from a 2013 machine
Give me a shout if you need any logs etc.