User tmkoeln describes in bug 10251 that the vnstat data isn't saved.
My investigation showed that there is general bug in backup.pl, which doesn't save a bunch of directories.
Browsing through git showed that  this bug is introduced years ago.
Therefore my question: Are there other "strange" issues with backup/restore?

If I fix this known bug it would help knowing about other abnormalities. ;)

A workaround for the moment is posted in bugzilla.

Regards
Bernhard