Pernille, I've had that issue and have managed to sort of fix it, but I don't recommend trying this without challenge board approval. When it happened to me it was a regular file, not a challenge file.
What I did was move the family out to an empty lot and save them to the library. Then I saved a copy of the now-empty house to the library. I then bulldozed the house, so it was an empty residential lot. I also bulldozed the family, removing them from town. Then I placed the copy of the house on the lot and the copy of the family in the house. If everything worked out properly the family still had all of their things, relationships, jobs, etc.
To be honest I had this happen frequently in an older patch/expansion version of the game, and it generally came from travel and it generally meant the game was already a little corrupted. There were times where I could fix the file, and times where the fix was temporary and the issues slowly started happening again. Hopefully what you're experiencing isn't this and you're able to fix your dynasty and carry on. Best of luck!