This program was not created in PowerPoint and cannot be opened. (also: PowerPoint found a problem with content — repair prompt on a Keynote export)

What's actually wrong

Keynote's PowerPoint exporter writes .pptx files that are close to the spec but not always conformant — certain grouped objects, images placed on master slides, unsupported fonts, and some transitions produce XML that PowerPoint for Windows rejects even though PowerPoint for Mac opens the same file fine. Nothing is lost: your real presentation is safe in the original .key file. The problem is purely getting a version out of it that Windows PowerPoint will accept.

Free fixes — try these first

1. Re-export from Keynote after a cleanup pass: ungroup all grouped objects (select all > Arrange > Ungroup) and remove images from master slides — Apple forum users confirmed both of these were the exact export-breakers.
2. If you have PowerPoint for Mac, open the exported .pptx there (it usually works), then File > Save As to produce a fresh copy — Windows PowerPoint accepts the Mac-PowerPoint-rewritten file. This is the confirmed fix from the Apple Support thread.
3. Upload the exported file to Google Drive, open in Google Slides, then File > Download > .pptx — Slides rewrites the package into conformant form.
4. Open the export in LibreOffice Impress and save as 'PowerPoint 2007-365 (.pptx)' — another rewriting path that a community member confirmed working.
5. On the Windows side, right-click the received file > Properties > Unblock before assuming the export is broken — emailed and downloaded files get blocked and show similar errors.

If the free fixes fail

If you only have the exported .pptx — the .key original is gone or on someone else's Mac — and no rewriting trick opens it, the file's XML is non-conformant in a way the mainstream importers won't tolerate. Our analyzer parses it against the actual OOXML spec, normalizes the parts Keynote wrote wrong, and previews how much of the deck reads back cleanly.