My word, I feel awful. DiapDealer first brought up images early on here, suspecting that may be the problem. At the time I glanced down my list of images and nothing looked amiss. (There
was a mix of images, from clipart png's to photo gif's and jpg's.) But an image was indeed the problem.
What I did:
1. Opened project in Sigil.
2. Went to Images folder, opened, and shift+clicked to select all
3. Saved As... to a new desktop folder.
Now here's where I knew there was a bad image.... when I pasted in the images to the dupe folder, most appeared instantly as thumbnails, but as I scrolled down the page a handful in the middle had the default icon for image, instead of a thumbnail, even though the window had been open plenty of time to load all thumbnails. I shift-clicked to select them, thinking they must be the problem, but a moment after I did that, their thumbnails appeared... then I noticed the top of the window had the green progress bar going, which only happens when the window has a problem loading whatever is in it. So obviously at least one image was bad, but not necessarily the ones that loaded thumbnails last.
So ... figured I'd first go through and delete all images I ended up not using in the project.
4. Minimized the dupe folder alongside Sigil
5. Looked at each file in Sigil Images folder by finding it's corresponding thumbnail in the duplicated desktop folder. When I got to an image I had not used in my project after all, I deleted it within Sigil using its delete feature [and in the dupe folder to make comparing easier]. I ended up deleting 74 files. And I noticed two of them were clipart pngs with exceedingly long names...the names were sentences with hyphens between the words. The only images left were ones I was actually using (129).
6. Saved Sigil then clicked the toolbar icon to insert an image, and the file insertion window opened (thumbnails were turned off)... turned them on and .... yup. Worked perfectly.
I suspected the long-named pngs so went to Trash and clicked to Restore the longest one, then went looking for it to add to Sigil, but I could not find it in either the dupe folder or within Sigil (in case trash restored it there). So next I went back to trash and instead of hitting restore, dragged the other long-named file to desktop, but when I did that the long name got truncated. When I clicked on it to rename, to see if the longer name would appear, my speakers lightly popped. I think it's pretty clear either one of both of these pngs were to blame.
I never noticed the names were long b/c everywhere I viewed the files the names were truncated. When I actually used a file, I'd rename it (and would have noticed), but I never ended up using those.
Will continue to test but I am sure that was it.
I can't tell you how sorry I am for all the work and time this caused. DiapDealer and KevinH, if you were here I would take you both to dinner and treat you to several libations of choice. As they said in Spartacus, "Apologies."
