The problem has been "solved" (moved): For the device of my DVD-burners there was "pktsetup" (for writing to DVDRAM) called via a initscript from which I dont know that it was aktivated.
This is misconception. pktsetup is not for DVD-RAM, but for CD-RW packet writing, but can as well be used with DVD-RW Restricted Overwrite and DVD+RW [at least as far as I understand].
Ok, I can now read my DVD successfully and completly.
So that packet writing module limits volume size to 4GB? Talk to the maintainer... readcd worked because it bypasses block device and issues raw read commands to its liking.
But it remains the question, why I should not read my my DVDRAM now completly...
You should not have any problems reading or writing DVD-RAM with pkt module disengaged. Nor should you have problems *reading* any kind of DVD media and writing with growisofs with pkt module disengaged. A.