Notable details we found so far:

• We need help finding the source code.
• Even outside of panoviewer, when we scale the image with MediaWikis's normal image scaling (which is based on ImageMagick) it breaks down at a certain size smaller than the 4000px needed by panoviewer.
  • 3940px works as expected.
  • 3941px stops scaling but instead crops the unscaled image.
• I tried but can't reproduce this issue locally.
• The other panorama works fine in all sizes, including 4000px.
• Unfortunately the 4000px seem to be hard-coded in the panoviewer code, so this is not a possible workaround.

Is this an ImageMagick bug? An out-of-memory issue? Can we find the error in a log?

There's clearly and underlying issue with the thumbnail generation in core. It's not an issue of the tool and it's hard to reproduce in core as well. It also seems that manually cropping the size of the panorama a bit might help to have a working version online. See T344233: Some custom generated thumbnails get massivily cropped.

Got solved by fixing the upstream issues in Thumbor \o/