I have had the same issue with 2013 but unsure why it does it. Try these
1.) Zip up the xml file and try importing. If you were using a different browser try it in IE.
2.) Open the xml and Excel and save as CSV and try importing now after configuring the data map. (this option might cause primary key differences for the newly created records)
Recently I have started using the data migration utility which is a part of the CRM2013 SDK that has been updated for CRM 2013 SP1 and found it really good. Try downloading the recent 2013 SDK from http://www.microsoft.com/en-au/download/details.aspx?id=40321
and navigate to the Tools\ConfigurationMigration folder and run the DataMigrationUtility.exe. The good thing about this tool is that it works in both 2011 and 2013 environments.