I can't figure out why I can't delete favourites in my app.

Here is an example:
favorites items 0 9999 want_url%3A1 title%3AFavoriten id%3A7dfc8ec5.0 name%3ABad%20Things type%3Aaudio url%3Afile%3A%2F%2F%2FF%3A%2FMusik%2Falbums%2Fjace %2520everett%2F01%2520Jace%2520Everett%2520-%2520Bad%2520Things.mp3 isaudio%3A1 hasitems%3A0 count%3A1

favorites delete item_id%3A7dfc8ec5.0

However the favourite still remains in my favourites list. I can add favourites just fine - I just can't delete them. If I use the webinterface I can delete them without problems.

Any idea what I am doing wrong?