I have solved the issue the following way:
First I export the backup job, delete it, and import it again. Get the same error.
Then I delete the backup job, reconfigure it manually, change the database path to the existing one: working.
Now everything is okay.
If it can have some interest for you, you will find below a screen print of the detailed error at 19:14 yesterday:
Thanks a lot @ts678 for your support.