ekkehard wrote:
  

perhaps the problem is that my BundleMarkerConverter class was not found -
all runs under OSGI
    

Yes, that is probably the cause of the problem. I have committed changes for 
better error reporting. 

Could you build logback from the trunk?
  

It should be as easy as:

  svn co http://svn.qos.ch/repos/logback/trunk/ target_directory
  cd target_diurectory
  mvn install

  

what do you think - should I add the import to logback core or logback 
classic ?
or could something else be wrong because the error notes that the 
conversion word wasn't valid
and not that class BundleMarkerConverter wasn't found
    

The error message can be misleading, hence my recent commit. The problem is most 
probably related to loading the BundleMarkerConverter class.