I figured out the problem: partially due to PEBKAC and partially due to missing static routes. I should also mention that the subnets in the isolated network were duplicates of our production network.
I did not fully explain to the user, for which this was built, how it was intended to be used. The other, and more important reason, was that a jump machine was configured with two vNICs: one inside the isolated network and the other outside that was routable to our production network. In order to communicate to the hosts inside the isolated I needed to add static host routes for each of the (dozen) hosts.
route /p add <ip of="" host="" on="" isolated="" network="">mask 255.255.255.255 <ip address="" of="" vnic="" on="" isolated="" network="">Route
http://technet.microsoft.com/en-us/library/ff961510.aspx
This meant of course that the jump machine would not be able to contact the production instance of the hosts we had in the isolated network, but that was not a real problem.</ip></ip>