I saw this error when you mix jar files Tridion SP1 and Tridion SP1#HR1 . You most likely (with confidence) have cd_core.jar from Tridion SP1 and not from Tridion SP1 HR1 .
Tridion SP1 HR1 is a prerequisite for Tridion UI / XM 2012. You need to replace all jar files from Tridion SP1 HR1 and restart the AppPool preview site. After copying, you will see the messages as shown in the log below, instead of ERRORS.
WARN AmbientDataContext - There is no current ambient data context - the ambient data framework is not properly initialised DEBUG AmbientDataContext - Setting current ambient data context: com.tridion.ambientdata.web.WebContext INFO AmbientDataConfig - Found claimStoreProvider definition in configuration file: com.tridion.siteedit.preview.PreviewClaimStoreProvider.
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