This upgrade was originally slated for 2015-05-09 but was postponed to the next day.
Tonight the HipChat server will be upgraded for an emergency patch. This upgrade will update the server to version 2.0 (from 1.4). Unfortunately there is no fix to the server unless we do this major upgrade. This upgrade will take around 1 hour and requires a reboot, which will forcefully log everybody out. For a list of all changes see: https://confluence.atlassian.com/hc/hipchat-server-release-notes-608731400.html