This error message generally indicates that the user's mailbox believes a message exists, but it cannot be found in the expected GRP file. This is typically because of GRP corruption. Given the version you are running, there is also a chance that it is related to an issue that was fixed in build 9413.
Fixed: Message counts that display on a folder don't match the number of messages.
If updating does not resolve the issue, then it's likely that there is something like anti-virus software that isn't properly excluding the domain directory.
Andrew Barker
Senior Software Developer
SmarterTools Inc.
www.smartertools.com