Workbench moderation state tab not displayed
Ah. Again, Fooled myself - it was a simple shoot into my own foot.
I had written custom PHP code in my costom theme in the function THEME_form_alter() to suppress the whole revisoning tab:

$form['revision_information']['#access'] = FALSE; 


When I then installed and configured the workbench module I didn't realize that it needs exactly THIS tab.

Works as expected now. Sorry for using your time... But maybe someone else learns from my errors...
Thanks for  your thoughts.