From my experience, this can happen if you copy the profile on old laptop while TB is running.

I'm absolutely certain that TB was not running while copying the profile. Is there a way to copy specific Local Folders from the old computer without starting from scratch? If I start from scratch, will TB messages from the old computer sync with the new computer, or will everything be lost on what I have done on the new computer?

Should be okay. With TB not running, use File Explorer to copy to the Local Folders section in profile.

Thanks David. That makes sense. Unfortunately, the old laptop and the new one are on different continents at the moment. I'll give it a try when I'm back on home turf and keep you posted.

since the inception of profile per install all sorts of issues arise.

These days I recommend replacing the content of the new install profile with the content of the the old profile. With Thunderbird closed of course. It is also probably a good idea to to delete the foldertree.json file to force a rebuild based on the new path location.