Error using CMIS in SharePoint 2013 with 2010 Experience Version

We have a home-grown application that uses CMIS to store documents in a SharePoint 2010 library. It has been working fine for couple of years until we decided to upgrade to SharePoint 2013. The SharePoint 2013 site was build as 2010 'Experience Version'  (14 Hive) and the 2013 CMIS was not visible under  the site collection feature so I downloaded and installed SharePoint 2010 admin tools and enabled CMIS for SP 2010. Now we are testing the connection and we are getting the following error:

‘MUST NOT contain a Document Type Declaration(DTD).’

Any ideas what this error might be. Did we go wrong in the process above? Any feedback will be very much appreciated.