Thanks JBasilio,
I might have created more problem tho... I forgot to mention the backups were on VTL tapes.
I did the command you proposed : http://i.imgur.com/TFjXpry.png
I choose a tape and it was, in fact, poor quality : http://i.imgur.com/LWH7Bc7.png
The tape had only objects from the session 2016/06/03-6 : http://i.imgur.com/nebNjvc.png
The verify failed saying the tapes was blank : http://i.imgur.com/k5pX5m1.png
Now the tapes appear blank : http://i.imgur.com/KFGv5zs.png
Can you tell me what went wrong ?
Thanks