Hi,
Backwards compatibility shouldn't be a problem for soft even with the missing plugins but just to make sure, backup the preferences of both older softimage and 2013 and than delete them so you have clean installations. Than try and load a simple scene (populated with a simple box for example) created with an older version into a newer one.
Have you tried re-installation? Or a famous runonce.bat fix? If everything fails, contact the support because this should definitely not be happening.