I found a level that crashes with the same Assert when the level is opened in the editor. There was no information about this level in the cooklog.
At first I opened all the levels individually and deleted all the crashing ones. By deleting those levels, I was able to cook. However, those levels could not be recovered.
I don’t know why the splines were broken, they were broken on multiple levels, so it could be triggered by editing blueprints used in common… .orz