All user specific page restrictions lost when upgrading to Confluence 5.2.4

Symptoms

When upgrading from Confluence 5.1.x or earlier to Confluence 5.2.4, the upgrade process erroneously removes all content permissions in the system, except for one for each user.

This is indicated by the log messages:

[atlassian.confluence.upgrade.UpgradeTask] removeDuplicates Found 315 redundant duplicates in the [USERNAME] column of the [CONTENT_PERM] table; deleting them
[atlassian.confluence.upgrade.UpgradeTask] deleteRows Deleted 315 rows out of 315 in CONTENT_PERM table

NB: Customers upgrading from 5.2.3 to 5.2.4 will not be affected by this issue. It only affects upgrade from 5.1.x and below.

Workaround

Affected customers should restore their data from a pre-upgrade backup, and run that until a version with a fix is released.

If restoring a pre-upgrade backup is not possible, please see this KB article for other options - if you have a backup from prior to the upgrade, the restrictions can be restored. We have created a plugin that will help with that. Alternatively, if you have no backups, the KB article contains instructions for how to find the affected pages, to update the restrictions manually.