Nero doesn't burn to DVD-R but does to DVD-RW

writing to a dvd-rw goes excellent, nero burns with 2.4 speed
But when I try to burn to a dvd-r, nero immediately gives an error: Burning failed at 2,4 speed, other speeds don’t work too.
I use normal 4x dvd-r disks and my burning device is aNEC DVD RW ND 1300A

anyone an idea?