My campaign would not load. I looked in the debug file, and found out that the game had problems finding a specific ship design, and when I looked in the ship design folder, it was not there.
So I opened the ship editor and created a new ship with the same name, and now the campaign can load again.
Normally it is not possible to delete ship designs that are in use in a campaign. I have no recollection of deleting this design, but I could have done it and forgot about it. Except that this is supposed to be impossible.
So I tend to believe either that the game somehow deleted it, which is a bug, or that I did it, in which case it is also a bug, because designs in use should not be deletable.
I am sorry I cannot be more specific.