I will join the discussion with an observation: Acronis is being stopped by files at appdata\local\thunderbird. Maybe someone here can explain the function of that folder. I deleted my appdata\local\thunderbird folder a few years ago and have not missed it. So, will this person be safe to just delete the folder? Or have I just had dumb luck by not having the folder all these years?

I don't know. These links may help:

https://support.mozilla.org/gl/questions/1488404

https://support.mozilla.org/en-US/kb/dangerous-directories-Thunderbird-account-settings

I've solved the original problem - the file (or it's directory entry) was corrupted and could not be accessed. It turned out to be the only one so damaged in the hundreds in the cache (A stray cosmic ray??). Once that was fixed with Chkdsk I had no further problem with Acronis.

A Thunderbird question - is it necessary to retain all the file (some years old) in cache2/entries?

Nothing Thunderbird stores in %Appdata%\local is required for a backup the path to Thunderbird default profiles that you should backup can be obtained by typing %appdata%\Thunderbird in the windows file manager path line. This falls apart if you use the Microsoft store version as it used Microsoft MSIX technology to force a new location for Thunderbird's profile into it's container. Sand-boxing is very popular, but in the case of Mozilla products basically without documentation and with plenty of bugs. Not just with Windows MSIX either, Linux flatpaks and SNAP packages also have issues with resource restrictions.

If you are intending to use Acronis you probably need to seriously look at the whole Microsoft shadow copy thing as it is another high fidelity backup scheme and it looks from the path in your image to be one of 30 or more backups that you are actively backing up. Personally I turned it and Microsoft search integration off on my old file system. I seriously improved the sytems storage and general responsiveness with a spinning platter drive.