I'm not certain, but I am pretty sure that this is due to the changes in the SolidWorks file format that were made in SolidWorks 2015. Read about that here. I don't have a workaround for you. You'd have to email API support about that. If they provide any useful information please post it here.
you may have found out by yourself that the file format changes (for why and how see link in answer of Keith Rice).
You can use the new Document Manager API 2015 to access from "outside".
For more infos see: 2015 SOLIDWORKS API Help - Third-party Data in SOLIDWORKS Files
Thanks Keith & Heino. I was aware of the change, just wanted to know the reason. SW support told me that it's simply for disk file size reasons (the format is zipped, though that's not to say that you can simply un-zip the file and get a pre-2015 file). Although disk is cheap, all the distributed processing through PDM systems still means that file size is important.