Windows 7 64 bit computer cannot rewrite DVD+RW media, whereas DVD+R media or fresh DVD+RW are handled just fine.
It looks like that it is not hardware or media related, for an external DVD rewriter connected to the computer (USB or 1394) gives the same results as the built in drive.
On another computer with Vista 32 on it, the external drive just works ok. Therefore it must be something in the W7 system.
Then I tried the erase and rewriting with Alcohol 120%.
The same DVDRW is erased an rewritten by this program just fine.
In this program the Default Device Control Interface has been used instead of the Windows ATAPI/SCSI Control Interface.
It is not very important because I can do the rewriting on the Vista machine, or convert the data to ISO. (Alcohol only writes ISO files).
However, it keeps me wondering where to look for a solution. Any suggestion to resolve this issue would be greatly appreciated.
The solution was simple, remove Alcohol and after that everything was working as advertised.
However, searching for conflict Alcohol and other burning software I found a better way to solve the problem: http://club.myce.com/f59/alcohol-conflict-nero-203075/
This way I can keep Alcohol alongside of other burning programs.