A form region cannot be opened. The form region manifest contains invalid XML.
Option 1
Uninstall ClearContext via Windows Control Panel > Programs. Reinstall v5.2.1 or later.
Option 2
Turn off "Show add-in user interface errors" in Outlook:
In Outlook 2010, this is accessed via Files > Options > Advanced. In Outlook 2007, go to Tools > Options > Other > Advanced.
Note that you will no longer see MessageContext in open items.
Option 3
If you are a developer and need to leave the above enabled, try removing your RelatedView registry keys. You will likely need to reapply this patch after each upgrade.