Created attachment 203226 [details]
ooo24019-1.ppt

The attached PPT presentation is from an old AOO bug report.
After converting to PPTX with Impress, PowerPoint fails to open it, and tries to repair the presentation.

OOXML-Validator gives the following result for each slide:
{"Description":"The element has incomplete content. List of possible elements expected: <http://schemas.openxmlformats.org/drawingml/2006/main:blipFill>,<http://schemas.openxmlformats.org/drawingml/2006/main:gradFill>,<http://schemas.openxmlformats.org/drawingml/2006/main:noFill>,<http://schemas.openxmlformats.org/drawingml/2006/main:pattFill>,<http://schemas.openxmlformats.org/drawingml/2006/main:solidFill>.","Path":{"Namespaces":{},"XPath":"/p:sld[1]/p:cSld[1]/p:bg[1]/p:bgPr[1]","PartUri":"/ppt/slides/slide1.xml"},"Id":"Sch_IncompleteContentExpectingComplex","ErrorType":"Schema"}

Officeotron also notes the problem in each slide:
ERROR      - (slides/slide1.xml:2 col:459) cvc-complex-type.2.4.b: The content of element 'p:bgPr' is not complete. One of '{"http://schemas.openxmlformats.org/drawingml/2006/main":noFill, "http://schemas.openxmlformats.org/drawingml/2006/main":solidFill, 
"http://schemas.openxmlformats.org/drawingml/2006/main":gradFill, "http://schemas.openxmlformats.org/drawingml/2006/main":blipFill, "http://schemas.openxmlformats.org/drawingml/2006/main":pattFill, "http://schemas.openxmlformats.org/drawingml/2006/main":grpFill}' is expected.

The problem is the following bit that gets written in each slideX.xml, removing them from the unzipped files and rezipping the result fixes the issue:
<p:bg>
  <p:bgPr/>
</p:bg>

Not sure why these were written in the first place, the background is coming from the master slide.
Interestingly, if the presentation is saved to PPTX, or just saved back to PPT in PowerPoint first, then the roundtripped file can also be opened in PowerPoint.
The presentation itself suffers some visual issues after roundtripping eg. starting from the PPTX, but that can be reported separately.

Observed with LO 26.2.0.0.alpha0+ (8ea8e254a3151f5390f3a10ff156fcaf8e7c5d5c), 4.0.0.3 / Windows.