Ok. I tested properly with CDBurner XP Pro and overwriting a second time a brand new DVD+RW 4x (without prior formating with DVDInfo Pro) gives absolutely no errors afterward.
Nero seems to be fine too so... Image for Windows is the culprit. It do not handle correctly DVD+RW overwriting I conclude.
Strange though that after a DVDInfo full format there's no more problems overwriting again and again with Image for Windows...!