Prior to 3.0 puppetdb.conf took entries for server/port. When we implemented terminus failover we changed that to server_urls and deprecated the old config. Eventually it was retired.
Now, when you use the old style config PDB doesn't read a server and instead falls back to the default name 'puppetdb'. This causes a confusing message in the server log because as far as the user knows, they've correctly specified the PDB host as something other than puppetdb.
Depending on what our options are, we should either crash puppetserver or watch for the old config style and log a message explicitly mentioning the new one.