[DirtyHarry]

I've found a workaround.

• I've deleted the song (at 12%). Not the last song displayed in companion ("backed up song ...") but the last song displayed at the tablet ("transfered song ...")
  
• I've also deleted the songs at 38%, 47% and 77% because backups failed (The songs have been successfully backed up since months).
  
• Then the backup succeeded.
  
• After that I imported the deleted songs again.
  
• The next
  

backup succeeded also.

I wonder why these songs made backups impossible. They where untouched since months.