To start with, I'll just clarify that this appears to be a different issue to the numerous migration ones already covered.
In short, I've migrated my site to a new domain (both in root, no directory changes) according to the WP manual, however now images in all of the pages are lacking their thumbnails & are simply displayed by their names. Clicking on their link/name displays the full image, as it should.
Looking at the page source, WP isn't even trying to insert the thumbnail - as if it thinks it doesn't exist!
The media uploader shows all of the images, but lacks any thumbnails & just shows a placeholder. Clicking on View takes me to the full, working image.
Using the Regenerate Thumbnails plugin, it spits out the error: "The originally uploaded image file cannot be found at " - at? Looks like it could be a hiccup in the DB, despite it still being able to point me to the original file.
New images upload & display without any issue.