I have the same issue, and found some info that didn't solve it for me but might for you.
The bug is that one of the three guild leaders drown when you come too close to him before opening the Crestwood dam (that's the theory at least). There should be other assassins out there who occasionally drop the same token, as the Crestwood assassins never respawn.
One possibility is HERE in the Wastes in Western Approach.
Another option is Valammar in the Hinterlands, more info on that option HERE.
Neither worked for me after hours of pointless grinding, so I stopped playing. But it apparently worked for others, so go ahead and try it!
