everything is working fine now. I didn´t recognize, that you have to delete the symbol in the first step and than the imported object. But - does it really make sense, to delete objects out of the library instantly, when I delete the instance of an object on the stage, resp. when I delete an symbol, containing the object, out of the library? May be, I want to use the object in an other way, create a new object and so on. At the moment, I always have to reimport it.
Thanks and best regards,