It worked..   
..after updating all of the three DCS installations, manually deleting the registry keys in   
HKEY_LOCAL_MACHINE\SOFTWARE\Leatherneck Simulations\DCS:MIG-21Bis 
 HKEY_CURRENT_USER\SOFTWARE\Leatherneck Simulations\DCS:MIG-21Bis    
!!YOU ONLY HAVE TO DELETE THE "Keys" FOLDER THERE AND NOTHING ELSE!!   
and finally activating the Mig-21 Bis module again with the original code 
that I've bought it with.   
Hope that helps at least some of you,   
Brgds Geraint