Can you please share the complete stack trace from the logs? From the error message you have stated, I see that the class invovled with the issue is "Assign-". Can you please elaborate how the class is used in your scenario?
Also, as per my understanding there must be some configuration issue for such error. So, it would be great if you can share the scenario where the issue is occuring to identify the root cause.
I would run Tracer on the command sequence and then see how a class of Assign- is being passed to the point of failure. It sounds as if the code is expecting a concrete class name, probably derived from Assign-. Did a page fail to open? Or is a parameter missing?
You may need to hard delete the assignment before changing the stage. I did it using pxDeleteAssignmentsForWork activity. I didnt like the approach very much. But I am not sure if there is a better solution.