Given this configuration:
[langfeldt.net;lookfar.langfeldt.net] address 127.0.0.1
If you add use_node_name yes then munin-update will snoop the munin-nodes host name from the connection banner:
# munin node at lookfar
(e.g., "lookfar" in this case) and use that to retrieve plugin names. In the protocol this takes this form
list lookfar acpi apt apt_all cpu cpuspeed df df_inode ...
If use_node_name is no then the host name from the master configuration stanza is used: lookfar.langfeldt.net. If the node itself thinks it is called "lookfar" this would retrieve an empty list:
list lookfar.langfeldt.net <empty line>
If the node does not know its correct host name then in munin-node.conf you can use the directive host_name <host name> to set it.
use_node_name no is the default and this should normally not be changed. The correct fix is to make the nodes know their correct host names.