Problem solved by Club Carlson Hotline and certificates worked. Problem is that the link given in the certificate leads to a misleading web site. You enter the eCert number early on and then its is never deducted from the price.
If you go via normal booking process, choose additional options, enter promo code, proceed with booking, at the bottom of the rate choice form there is a link you have to click to enter the cert nr. Then everything works.