Sprint data gone after combo JIRA+Agile upgrade [Solved]

Standard

Yesterday I did dual upgrade JIRA Agile (6.4.5 -> 6.6.0) followed by JIRA itself (6.3 -> 6.3.6) and everything went alright without any error. After the upgrade JIRA Agile components started to throw errors – mainly general errors and

clauseValues is empty

errors mostly in places where the issues are listed (sprint backlog, reports etc.). The errors effectively rendered all sprint data not available for any of previous sprints.

The problem can be fixed if you have backed up JIRA database before proceeding with the upgrades):

  1. Downgrade JIRA Agile back to 6.4.5 – Remove addon in Addons manager (don’t be afraid, the settings are saved), download older version from Atlassian Marketplace and install it again manually from file.
  2. Stop JIRA
  3. Drop JIRA database and restore previous database content
  4. Start JIRA again (and watch the log file while starting to be sure the database will get upgraded automatically)