So: The error seems to be that the keyboard assistant expects qwerty. It must have run some time, and it wanted me to press the button next to the left shift key, which on svdvorak is "ö". I had to delete the library/preferences/com.keyboardsomethingsomething and redo the keyboard identification and press the "<" key (which on svdvorak is about as far away as you can come from the ö key).
Now it works.