Hmmm
Even with an empty drum?
The machine has a catch that basically holds the door closed.
That is spring loaded.
You see a hoop on the door itself, and a black catch on the lock. Closing the door swings the catch closed, catching the hook.
When the door locks, basically, only that catch is physically locked.
Now, that catch usually is spring loaded and has 2 stable states - open door, catch open, or closed and catch closed.
I have never heard that that system breaks, but still is abled to lock when closed.
Usually, the door just wont lock at all.
But there isn't anything else holding the door closed while not locked. So it can only be the door lock.
Maybe try activating the manual door release a few times and see if anything got stuck there.
That can basically override all door locking mechanics, allowing the door to swing open what ever be the case.
If you do have warranty, yeah, sure, use it.
If not, replacing the door lock is somewhat annoying, but not difficult.
Go to mieleusa.com -> Support -> Parts & Accesoriea -> Spare Parts Catalouge (midway down the page) .
Searching for the WXR860, on the second drawing (030), part 200 is the electromagnetic door lock which is surprisingly affordable at 90$.