I can't understand this:
the same optfs tar archive was able to be restored without any error two days ago.
Today, it gives several hundreds of "gtar: path/to/file/name: Cannot open: Stale NFS file handle" errors.
Tar exits with "Error: returned 2."

What's happening there?

EDIT: which way can I determine if the flash chip hosting /opt (and MyDocs) is becoming broken? This might be a common explanation of all my problems.

EDIT: weirdly enough, to restore my optfs, I had to:
1) restore the opt img with dd (but it wasn't good, when rebooting the optfs was mounted read only)
2) restore it again from tar.
This time no NFS stale handle errors

If I tried to restore the tar without putting there the img before, I had countless NFS stale handle errors and the optfs was damaged.

I am more and more convinced that my optfs chip is broken.