** Message: virtstyle ipliteral
** Message: connect from 127.0.0.1, assigned file is /home/user/fs/image
** Message: Authorized client
** Message: Starting to serve
** Message: Size of exported file/device is 33554432
Negotiation: Error: Server closed connection
Error: Read failed: Connection reset by peer
Apparently this simply does not work.
Solution was to create the server config file and leave out the server command line options completely.
exportname = /home/user/fs/image
authfile = /etc/nbd-server/allow
Then run the client with the name specified in the config.
sudo ./nbd-client -name export 127.0.0.1 10809 /dev/nbd1
I could have saved some time debugging this if I just had read the this bug report thoroughly enough: