Created attachment 203551 [details]
forum-mso-de-38427.ppt

Open and save the attached file, forum-mso-de-38427.ppt, to PPTX and try opening it in PowerPoint, it will fail.

Interestingly, when converting using convert-to with a command something like the following, the result is OK:
soffice.exe --convert-to pptx forum-mso-de-38427.ppt

The meaningful difference between the files is in '_rels/.rels' after unzipping, where the convert-to result has this:
<Relationship Id="rId1" Type="http://schemas.openxmlformats.org/officedocument/2006/relationships/metadata/core-properties" Target="docProps/core.xml"/>
While after saving from the UI as PowerPoint 2007-365 (*.pptx) format, it has the following:
<Relationship Id="rId1" Type="http://schemas.openxmlformats.org/package/2006/relationships/metadata/core-properties" Target="docProps/core.xml"/>

When saving using the UI, there's another format, Office Open XML Presentation (*.pptx), this also produces a working file, and the relationship type URI is the same as in the convert-to case.

What's interesting is that PowerPoint also exports this part like the "bad" PowerPoint 2007-365 (*.pptx) format from the UI, and of course it's able to read the file back, which means there must be other relevant differences:
<Relationship Id="rId2" Type="http://schemas.openxmlformats.org/package/2006/relationships/metadata/core-properties" Target="docProps/core.xml"/>

Observed with LO 26.2.0.0.alpha0+ (8ea8e254a3151f5390f3a10ff156fcaf8e7c5d5c) / Windows.
In latest of 5.2 Windows bibisect repo the export is OK, in 5.3 there is a likely unrelated change, causing validity issues (caught by OOXML-Validator), and when that disappears in 6.2, the file remains invalid (though OOXML-Validator shows no validity issues). Probably not worth chasing, and instead best to investigate this independently.
Since exporting most presentations to PPTX is OK in Impress, it might be interesting what detail of this presentation matters.
Based on the initial observation (UI vs. --convert-to), bug 165180 might be relevant.

Attaching PPTX exported by PowerPoint 2013 for reference, both the regular, transitional and strict ones.