So ! i've tested further B1 system backup with big upnp library.
I've tried to add files, folder by folder, in upnp library.
When it grows around 4000 or 4500 files, "system backup" file ended to be 0k.
In fact, the more upnp library grows, the more the backup file grows too... (That seems logic).
And the biggest file that i got was close to 500ko. Then, it went 0ko.
Is there some kind of limit for backup file size (around 500ko)
This could explain the eratic behaviour with system backup....