You mean clues for the developer, right?

Not looking, I think. Had they been they would have responded by now. Could be the developers are no longer around? That would be quite sad.

Nothing changes. Still the same. Maybe it is some change in Google Drive API that broke T.O.  ?

Yes, it is a problem with restore feature as far as I can tell. Backup itself is OK.

One question arises, though: what good is a backup if you cannot restore it later :)

Same problem here. RESTORE not working any more. This is new - several days ago everything worked as expected.