I have several problems on this bike. Here are the elements that have failed and other pieces of information:
- the chain has derailed past the largest sprocket
- the rear derailleur has rotated 180 degrees, and is caught in a spoke (between the frame of the derailleur and one of its sprockets)
- half of the derailleur is now bent 45 degrees from its frame (derailleur on picture 2 should be U-shaped, not V-shaped)
- sprockets showed some signs of wear, but not to the point where it causes the chain to skip when load is applied
- the threaded hole to which the rear derailleur is screwed shows some sign of wear ( this is a modular part which is itself screwed to the frame)
- rear axle is broken in half right in the middle. I don't have the tools right now to open it, I believe this requires a punch and a hammer to remove the cog set.. Right now I have already lost some bearing balls after I moved the axle from the dropouts.
Now I'm wondering if there is a single failure that could explain what happened.
Thanks to the answers, I inspected the derailleur closely and found this: There is a missing chip exactly where the L screw is supposed to hit the frame. Apart from that and the fact that it's bent beyond repair (IMO) the derailleur is actually brand new..
Here is a picture of the broken axle I knew something were really wrong when bearing balls started falling on the ground...