I have the same problem. Can't export from Keynote 5.0.5 to PPT 12.2.8.
Actually, unfortunately given how Microsoft plays, it is Apple's problem. Microsoft doesn't need to read Keynote because they want everyone to use PowerPoint. If Apple wants people to use Keynote, they need to be able to export to PPT or PPTX. If Microsoft has screwed that up, Apple needs to respond and figure out a way to make it work. Microsoft certainly won't make that work.
As much as I'd love to use Keynote, if I can't reliably share with PowerPoint, then sadly I will continue to use PPT, which is a huge shame.