This is just a small reminder for me and everybody else who gets fooled like me:

If the loopback device (lo) is not up, you cannot ping localhost!

This is true for all local addresses, even though they are not on the loopback interface! It's kind of confusing, because everybody else can ping the box, only Linux cannot ping (or reach anyhow) localhost. So the quick solution is

ip link set lo up

This problem has been solved and described on other sites as well, but if this one helps you not to spend three hours debugging your nfs setup, it's worth populating the net with yet another article regarding this problem.