It appears to be some kind of self locking nut with a square washer that seems impossible to get to. From what I can tell, the PO has not threaded the bolt properly, so it got stuck halfways. Any ideas on how to fix this? Since the bolt needs to be adjustable to be able to adjust the frame, I guess I cannot weld a new bolt in place. And even it I could get the bolt out of the washer, I'm not sure the old washer will work again. And how the h**l do I get to the washer

I have added a few pictures of the disaster, any help really appreciated:
Before cutting the bolt

After cutting

