I have found, what the hell was wrong:
I had 2 Serial Numbers for Nero 5.5
One of my old Traxdata 8x 4x 32x CDRW
and one of my new 16x 10x 32x LiteOn CDRW
The LiteOn Key supports Nero and Nero Express and the Traxdata only supports only Nero Express.
When i´ve typed in the Traxdata Serial i can only start Nero Express - even when i click on the Nero (NOT NERO EXPRESS) icon!
Hope that will help someone who has the same Problem!
I found out, that the one Key was an OEM one and the other is a serial for the Normal Version.