Have an antivirus program scanning in the background? There can be many causes of file contention, which this is an example of, Antivirus is the most common on windows.

It could also be Thunderbird, but I would only expect that is you have multiple instances running at the same time and a number of checks on the profile failed.

anti-virus: Only Windows Defender. which has been actively and non-problematically operating together for months. No recent changes/updates.

file contention: Not from multiple instances. I've rebooted several times, and Thunderbird does not auto-start. However, I don't know which file(s) are locked/in contention. It's possible a file was opened/locked during an unexpected system crash and was left in a locked state. But I don't know how to determine which file it is.

I don't have anything concrete, but try putting Thunderbird into offline mode and try it.

That is the ((o)) icon beside the word done on the bottom left of your image.

Put in offline mode: failed, same issue

um. Is the profile you are trying to import the same one that Thunderbird is using?

Why are you importing it anyway?