Error is displayed on Standard client when opening the users mailfile.
To fix shut down the Notes client and then delete the following folders under the Data folder:
Data\workspace\.metadata\.plugins\com.ibm.portal.app
Data\workspace\.metadata\.plugins\com.ibm.rcp.portal.app
Restart the Notes client.
Thanks to Stephan Bovet for his post on the 8.5 forum for this solution.