Are you running Share on the same Tomcat instance as the Repository? (If you use the default installer you are)
If you have enabled SSO, did you correctly update all the "Remote" endpoints in share-config-custom.xml to use the /wcs endpoint? Even the new endpoint (not listed in any documentation) that is provided by the alfresco-share-services module called "alfresco-api"?
After investigating our endpoint configurations & updating them to include the latest alfresco-api endpoint & updating the connector classes to org.alfresco.web.site.servlet.SlingshotAlfrescoConnector from org.springframework.extensions.webscripts.connector.AlfrescoConnector, the continuous 'Authentication required' popup is no longer occurring, however our SSO via CAS is still incorrectly functioning. What appears to happen now is upon entering the URL of the site's share services one is redirected to CAS, and must enter username/password as normal. upon successful verification by CAS an 'Authentication Required' Popup shows up once, and if dismissed, the user is redirected to the internal Share Login page. Reentering the username/password allows access to share services as normal.
We believe this current error to be related to CAS Configuration. We are using the cas-client-core-3.3.3.jar, and while we are aware that is isn't the officially recommended method to use CAS, we'd rather not have to change to a different cas client implementation at this time if we don't have to, as we have used it without issue with version 5.1.1. Providing the latest JMX dump & catalina.out, as well as the share-config-custom.xml and web.xml files from alfresco & share for investigation should they prove helpful.
Once again, any assistance in resolving this issue would be greatly appreciated.
Did you file a ticket with Alfresco Support about this? I know that various people in the community have had issues with CAS support in 5.1/5.2, but I personally have not used any CAS solution in ages with Alfresco...