May be that this was one problem - but not the main problem. Changing to 1280x1024 will have no positive result. The message is still "only one device....; disabling Twinview". But it seems that I'm not the only one whith this problem. (see above).
Searching the net, I found a notice that nvidia ?? change the TV chip for this card, but no information which chip is used.
any more idears ??