I tried testing the issue with Monarch 11,12 and 13 and they all gave the same error message that "The structure of the data column does not match the export data set".
For further testing would it be okay to ask for structures of the database you are trying to export, or maybe the report and model files if possible.
Your reply prompted me to look at the Access table as being the cause of the problem. I found a Datawatch thread that said V 12 use's Access 2010 format and a solution is to re-create the Access table in 2010 accdb vs 2003 mdb extension. I think that will work. Will reply with the results.