Using Outlook 2007, one member of staff is plagued by this error, it occurs whenever she goes to read an email message that has previously been read.
We have checked there are no add-ins that are not meant to be there.
There are thousands of entries on the web if you Google this error, but no solutions applicable to Outlook to get rid of it.
We are not able to start Outlook in Safe mode, as our apps are delivered via Citrix. We have 3,500 other users getting Outlook via Citrix, no one else gets this error!