Found the solution for my problem:
1) As written in my answer to Toms post, I had in typo in the command line ... stupid.
2) The quotes regarding the command line call had been misplaced (Toms answer)
3) I found out that running 'unopkg.com' with the 'shared' parameter is only executed when the command line is being run with administration rights. I thought that the attribute 'impersonated="yes"' in my CustomAction would be enough, but it didn`t help.
Guess I have to dig deeper into documentation of WiX regarding UAC.
Also thanks to Ralf. I didnt try his solutions, but you might give it a shot.