Nothing, the airline ended up accepting it as regular luggage. The problem was getting it to the airport. We tried to get it in various cars and it wouldn't fit in anything, not even a boxy Volvo station wagon. We ended up strapping it to the roof of the Volvo and the GF was driven to the nearby train station while I drove to the airport to help out once she arrived there. She nearly got kicked out of the train station, but she can have such a helpless look to her that they let her through.
Next time, no bring on train! :angrygerman: