I ran across this today and the fix is simple.
I have a powerpoint that's a .pptx PowerPoint file. That means it was created in PowerPoint 2007 or later. Now, .ppt is for earlier versions than I think 2007. A client sent me a link to a PowerPoint that was to be used in an edit and for some reason QuickView nor Keynote could open it. Keynote threw out an "Import Error".
I then noticed that the file was filename.pptx.ppt when it should have just been .pptx. Somehow the download service that they used to send me the file for some reason appended on .ppt to the file.
Changing the file from .pptx.ppt to it's original .pptx file extension solved the problem of opening it in Keynote.