Hi Rainer, Did you sign your satellite assemblies with the same
public-private key pair?  If the main assembly is signed, the satellite
assemblies must be signed with the same public-private key pair.  If you
have already tried that, then you may be able to find loading errors by
checking the log in fuslogvw.  Check Suzanne Cook's blog entry for more
information on how to debug loading failures.  
http://blogs.msdn.com/suzcook/archive/2003/05/29/57120.aspx
Your version numbers should also be the same, however, you can update your
main assembly without updating your satellite assembly by setting the
satellitecontractversionattribute - see more info here:  
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/cpref/html/
frlrfsystemresourcessatellitecontractversionattributeclasstopic.asp
Hope this helps,
Marin Millar [MSFT]