Writing a DWD+RW under Linux should be simple, something like:
$ wodim speed=2 dev=/dev/sr0 -v -data ubuntu-16.04.3-desktop-i386.iso
However, in my case it fails. Some background knowledge suggests to format the disc first, but blanking the disc fails too.
Solution is: format the disc with another utility:
$ dvd+rw-format -blank /dev/sr0
After this step, writing does work.