Originally Posted by
CMAW
Definitely from behind the dropout. The most fragile part of these derailers is in fact how the tabbed washer is attached to the end of the sprung pivot bolt. It's easily forced, making your derailer useless. To be on the safe side, use 2 allen keys, one to loosen (inside of the dropout) and one to keep the pivot bolt steady.
Bingo. Something I learned the hard way. However, a local welder made what appears to be a very credible repair to the part, and so far it's working just fine.
[edit] I mis-read your post. I gather you are saying the hole in the washer gets rounded out. In my case, the 90 degree tab snapped off. The hole that press-fits on the bolt was fine.