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