Wrote another dvd+r dl in dvd decrypter -> disc works in the LG 4120B now, booktype is dvd-rom.
Seems like it remembers the setting made in Nero.
However, obviously Nero is to be blamed for making that coaster which the LG 4120B could not read back itself
Unfortunately, all other drives detect a wrong disc capacity. The capacity read by the toshiba 1712, the pioneer a06 and the liteon 165 is equal to the disc's total capacity, not to the used capacity. When reading back the image in the toshiba 1712 for example, there are some zeros added between the end of he 1st layer and the beginning of the 2nd layer. This bug did not occur with firmware N0AD, so it's a flaw in A104, and not in the other drives.
As i'm out of dual layer dvd+r blanks at the moment, someone else might check if recording a disc entirely, i.e. using each sector, circumvents this bug. You can pad an image using this