For the current installation:
zp_options.image_size == 780,
zp_options.image_use_side (basic theme) == height

That would suggest the values were correctly set by the UI.

Indeed, and most certainly these are also used correctly.

Well, not here.. the image is scaled to 780x161.

I meant they are used by the code but somehow the outcome on your server is something wrong regardless…

I've given up for now and replaced image.php with the changed version using the custom function.

Is the version with the new thumbnail logic done yet?

Simple workaround for the Pano album: I will upload a 3:2 version of the image and mark that one as unpublished. Thumbnails of unpublished images still get displayed in the index.

Is the version with the new thumbnail logic done yet?

No

Thumbnails of unpublished images still get displayed in the index.

Unpublished items are only shown if you are loggedin or know the lin but setting as a album thumb should work.

Yes it works fine, I have tested it. Bit of a kludge, but for a single album it will do.

"New" gallery is now live (no more gallery.new). I disabled ImageMagick again because it made thumbnail generation quite slow.

New installation looks like an almost exact copy of the old gallery now. Thumbs up.

Great you managed it working to your liking. Btw, I am working on the thumbs stuff. Uses the same code base as the large images and I so far didn't encounter anything that would explain your issue and why we can't reproduce it…

I disabled ImageMagick again because it made thumbnail generation quite slow.

Interessting, heard that before. Actually the saying always was that Imagick was faster and less memory consuming than GD. When testing I personally didn't notice actual differences.