Created attachment 203188 [details]
Original - forum-mso-de-10080.ppt

Open the attached presentation, forum-mso-de-10080.ppt, and save it as PPTX, then open the result in PowerPoint.

=> PowerPoint encounters a problem, and removes part of the content.

When running the file through OOXML Validator, this is the result:
[{"Description":"Relationship tag requires attribute 'Target'. Line 2, position 86.","Path":null,"Id":null,"ErrorType":"OpenXmlPackageException"}]
(officeotron found no issue with the file)

If one:
- unzips the saved PPTX,
- executes the following command (can be done in Linux/Cygwin, needs xmllint installed): find . \( -name "*.xml" -o -name "*.rels" \) -type f -exec xmllint --output '{}' --format '{}' \;

...they can see that in ppt/slides/_rels/slide1.xml.rels (and others), there's the following relationship, with the empty target PP complained about:
<Relationship Id="rId1" Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/slide" Target=""/>

Checking slide1.xml a level up, the corresponding element is: <a:hlinkClick r:id="rId1" action="ppaction://hlinksldjump"/>

Saving the PPT as PPTX in PowerPoint, and similarly looking at the XML shows this instead: <a:hlinkClick r:id="" action="ppaction://hlinkshowjump?jump=nextslide"/>

With the following manual changes to the buggy file, PowerPoint can open it:
- remove Relationships with empty Target (slide1, slide3),
- replace 'r:id="rId1"' references in <a:hlinkClick ...> with 'r:id=""',
- in case of jumping to next slide, replace 'action="ppaction://hlinksldjump"' with 'action="ppaction://hlinkshowjump?jump=nextslide"'

Note that jumping to relative slides isn't supported in Impress, only to specific slides, it would be nice to add that, but not necessary for this bug.
PowerPoint offers the following options: First Slide, Last Slide, Next Slide, Previous Slide
The corresponding jump suffixes: firstslide, lastslide, nextslide, previousslide

Saving a PPTX with similar jump actions produces correct results.

Observed with LO 26.2.0.0.alpha0+ (8ea8e254a3151f5390f3a10ff156fcaf8e7c5d5c) / Windows.
In old versions, eg. 4.0.0.3 the link sipmly gets lost on export.