I'm trying to cut a piece out of an object I made by extruding a plane several times. As usual, I'm using a boolean difference, but for some reason (and it's not the first time this happens to me), the result is "incomplete", since not enough material is removed (see picture). When this happened in the past, I could solve the issue by using the "fast" setting on the modifier. Here, it makes it even worse (not material was removed at all, no matter the setting).
P.S.: Link to project