how can the leak be fixed? a different washer or ?

you have to be careful when putting this wand on not to X thread it, which is easy to do. The threads are plastic and it can happen easy if this happens it' will probably always leak. This happened to me and once I got it back on correctly I did not take it off again.

Is the leak at the hose end connection? Maybe a thicker washer? If that doesn't fix it, I would return it. If it is leaking around the adjustable nozzle, itself, probably not.