This error occurs due to the file type of the image. To resolve this issue, please open your Aperture Preferences and click on the Export tab. Change the External Editing File Format from PSD to TIFF (8 or 16 bit). Then, close your Aperture Preferences and open an image once more.
While this will prevent the issue from occurring going forward, images that have already been edited in a plug-in and are currently PSD files will need to be duplicated before opening it into another plug-in.