After trying to use jumbo frames on my newly upgraded network and realizing that one adapter is not enough, I simply left things as they were. Come today, my NFS mounted directories on the server were unaccessible and none of restarting the NFS server (just the NFS server process, not the machine itself) unmounting the directories or trying to mount them again helped. Eventually a little playing around with tcpdump on both the server and client

tcpdump -i any port 2049

revealed that the client was sending requests and the server the appropriate responses, but for some reason the responses were not reaching the client. Quickly disabling the jumbo frames on the server

ip link set mtu 1500 dev lan

and everything was back in order.
Conclusion: Jumbo frames are evil.

Update: The Gentoo Linux Wiki has a nice tip about jumbo frames and NFS and why it is not good to mix jumbo with standard frames.