I have a friend at SAS who told me about the problem and why it appears. It mostly show up when one tries to book a connecting flight (or codeshare), like KLR-ARN-OSL-FRA (this was the problem I had). The campaign fare system shows that there actually are seats available, but once it comes to confirmation its not. According to my friend its a certain booking class that is booked and when the system tries to create the complete itinerary it failes due to lack of free seats on one or more legs. This is no problem if you call the internet support, they will help you to book in a different booking class (and they honor the fare rate if you tell them what rate the site said).

SAS says they are working on this but no solution yet.

Gustaf