The DTS version here is the version of SQL Server Integration Services Designer within Visual Studio. I'm using VS 2017 and have version 14.0.3002.92, which is a couple of versions behind the latest version of SSIS for VS 2017. Version 15 is for VS 2019 (Marketplace); which i just checked and is still in preview.
Normally extensions are backward compatible, so it surprises me that you cannot open the file if you changed it; but older versions can't open files from newer version (or they may behave unexpectedly). Perhaps the user using version 14 has opened the file you modified in version 15? If so, then that may well have done irreparable "damage" to the file; and you'll need to get a old version from SVN. As, however, IS 15 is in preview still, then it could well not support older packages at the moment, or certain features. The above link I gave does also state "This is a preview release. We do not recommend using it for production."
Either way, however, you should ensure that everyone who is contributing to a project is using the same version of the extension and Visual Studio across the board, to ensure that problems like these don't occur. So unless you need support for SQL Server 2019 (Preview), I would stick to VS 2017 and SSIS Version 14 (which isn't in preview).
Excuse my typos and sometimes awful grammar. My fingers work faster than my brain does.