Looks like it was the Advanced Custom Fields plugin. Luckily, I ended up not needing it for this project, but it is a widely used plugin… You may want to test what the incompatibility is for future versions of this theme and others.
I actually discovered a solution: setting the size of the image to “Large” or “Medium” in the module settings can fix this. However, I still do not get the scrolling feature to browse images while in lightbox mode…
