Hi,
please set the PDFCreator logging to "trace" in the debug tab of the PDFCreator application settings.This should indicate why Ghostscript is unable to convert the PS file. Please update to the latest version of PDFCreator, if you haven't done so already. Older versions still call Ghostscript with the parameter FastWebView, which causes Ghostscript to crash for unknown reasons for certain PS files.
If the PS was generated from an encrypted PDF, Ghostscript will refuse to redistill it ot PDF, in which case the only option would be converting it to an image first.
Best regards,
Robin