Thanks a lot for your answer Francksoy.
I noticed one more thing: I must absolutely erase the medias with DVDInfo Pro. Erasing with other software or erasing via Image for Windows when the backup starts (Image as a built-in erase function allowing to reuse media without blanking it before) gives me a "corrupt image stream" when I validate the backup.
Even more, if I erase the DVD+RW's with CDBurner XP Pro or Deepburner, it causes strange things like DVD burner refusing to eject the tray or LED staying busy... Also, in these cases (erasing with these two apps), the media is not really erased and I can't write anything on the discs.
Probably a picky DVD burner when using DVD+RW or a firmware problem... or like you said, a built-in annoyance of some particular applications.
I'd bet it's a built-in annoyance of this particular program, asking you "if you want to erase the media" as soon as a RW is detected, would it be used or blank.
If the media is brand new, there's no warning message and all is fine. Only when reusing a DVD+RW.