The problem is not the files, so you may consider contacting PTC support.
Looks like the converter is not able to find your Mathcad15 installation. This may be fixed by trying to reinstall Mathcad 15.
You should use the latest release of Mathcad 15 which is M050 (released november 2017).
Your third file uses a reference to another file containing extra units. This would have to be converted, too, for the file to work. As far as I see the only unit used from that file is kNm which could easily defined from anew at the top pf the sheet.
