Exchange IMAP4 connector – Error Event ID 2006

A couple of users in my organisation use IMAP4 to connect to Exchange 2007 (Update rollup 9 applied) because they prefer Thunderbird / Postbox clients. One of the users is generating errors in the Application Log as follows:

An exception Microsoft.Exchange.Data.Storage.ConversionFailedException occurred while converting message Imap4Message 1523, user "*******", folder *********, subject: "******", date: "*******" into MIME format. Microsoft.Exchange.Data.Storage.ConversionFailedException: Message content has become corrupted. ---> System.ArgumentException: Value should be a valid content type in the form 'token/token'
Parameter name: value
   at Microsoft.Exchange.Data.Mime.ContentTypeHeader.set_Value(String value)
   at Microsoft.Exchange.Data.Storage.MimeStreamWriter.WriteHeader(HeaderId type, String data)
   at Microsoft.Exchange.Data.Storage.ItemToMimeConverter.WriteMimeStreamAttachment(StreamAttachmentBase attachment, MimeFlags flags)
   --- End of inner exception stack trace ---
   at Microsoft.Exchange.Data.Storage.ItemToMimeConverter.WriteMimeStreamAttachment(StreamAttachmentBase attachment, MimeFlags flags)
   at Microsoft.Exchange.Data.Storage.ItemToMimeConverter.WriteMimeAttachment(MimePartInfo part, MimeFlags flags)
   at Microsoft.Exchange.Data.Storage.ItemToMimeConverter.WriteMimePart(MimePartInfo part, MimeFlags mimeFlags)
   at Microsoft.Exchange.Data.Storage.ItemToMimeConverter.WriteMimeParts(List`1 parts, MimeFlags mimeFlags)
   at Microsoft.Exchange.Data.Storage.ItemToMimeConverter.WriteMimePart(MimePartInfo part, MimeFlags mimeFlags)
   at Microsoft.Exchange.Data.Storage.ImapItemConverter.<>c__DisplayClass2.<WriteMimePart>b__0()
   at Microsoft.Exchange.Data.Storage.ConvertUtils.CallCts(Trace tracer, String methodName, String exceptionString, CtsCall ctsCall)
   at Microsoft.Exchange.Data.Storage.ImapItemConverter.WriteMimePart(ItemToMimeConverter converter, MimeStreamWriter writer, OutboundConversionOptions options, MimePartInfo partInfo, MimeFlags conversionFlags)
   at Microsoft.Exchange.Data.Storage.ImapItemConverter.GetBody(Stream outStream)
   at Microsoft.Exchange.Data.Storage.ImapItemConverter.GetBody(Stream outStream, UInt32[] indices)

From my reading around it seems that the suggestion is to ask users to log in to Outlook / OWA and view the messages there. However, having logged in as the users myself, the messages cannot be found either through searching or by browsing the folder detailed in the log entry. The server returns the following error to the client:

"The message could not be retrieved using the IMAP4 protocol. The message has not been deleted and may be accessible using either Microsoft Outlook or Microsoft Office Outlook Web Access. You can also try contacting the original sender of the message to find out about the contents of the message.

Retrieval of this message will be retried when the server is updated with a fix that addresses the problem."

Messages were transferred in to Exchange by copying them from the old Apple Xserve, accessed using IMAP.

So my question, finally:
1. Is there any way to get the IMAP Exchange connector to rebuild its cache of messages since it doesn't seem to be pulling them directly from the MAPI store?
2. Alternatively, if there is no database, any ideas on why these messages don't appear in Outlook or OWA would be gratefully received.