I'm using Captivate Classic and I am unable to upload Powerpoint presentations that are in PPTX version. The PPT version works just fine. How can I get Captivate Classic to work with PPTX files?

If I recall correctly, Captivate Classic and earlier just convert the PPTX to PPT in the background anyway. Not the best option, but probably why the new version can actually convert the PPTX rather than just link to it. (I always had trouble editing the PPT directly in Classic and earlier. It worked when I edited the PPT directly in PowerPoint.)