I get pretty far with building my level. Reloading the editor seems to make the playable character appear... And then everything sort of works. But when I continue to add things, then at a certain point the editor keeps getting more and more read & write errors. So much that the program crashes.

I'm amazed that Larian could build a whole game with such a buggy editor... I'm thinking they were probably using a different version than we are using. It's also suprisingly user-unfriendly and complex to learn.