I have checked some more: The camera raw plug in is installed / saved at the correct directory:
C:\Program Files\Common Files\Adobe\Plug-Ins\CS3\File Formats
There are two old plugins in the same directory. If I rename the new and turn on the previous version, the "convert to DNG-button" is not disabled.
Then I get the feeling that the problem is with the new camera raw plugin (version 4.5).
I am thankful for all suggestions. Maybe someone from Adobe know something?