Symptoms

Users can't log in to the instance after upgrading.

Cause

Step 6 of Delegate user management to use JIRA logins was skipped. The upgrade guide describes moving <confluence-home>/confluence/WEB-INF/classes/atlassian-user.xml from the old instance into the new.

Resolution

Modify atlassian-user.xml to point to the osuser repository:

...
<osuser key="osuserRepository" name="OSUser Repository"/>
...