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With SSO configured and the ConfluenceCrowdSSOAuthenticator enabled in the seraph-config.xml file, install SU for Confluence and attempt to SU to any user.
Primary (non public) Atlassian Support
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Cases:
Atlassian Devloper Ticket:
Resolution
Issue has been resolved and a fix has been released in the following versions:
SU for Confluence v2.5.7 for Confluence 7.x
SU for Confluence v3.0.5.1 for Confluence 8.5.x
SU for confluence v3.0.6 for Confluence 8.6.x