Maintenance
From YPPedia
An example logon failure message
The Yohoho! Puzzle Pirates game client will occasionally report that the servers are undergoing maintenance. Sometimes this means that the server code is being updated for a new release or a bugfix. At other times the game database is undergoing an integrity check to guard against data corruption and possibly undergoing a restore to a known good state. Another possibility is that the server is operating fine but some game clients for whatever reason are unable to log on (check the server status page in the external links section).
