If you are having problems reassigning an Opportunity or an other entity, ygis might be caused by the related entities attached to the Opportunity.
Have experienced this in several occations:
This error is caused by the N:1 relation between the Opportunity an the custom entity (new_op...).
The relationship behavior of Assign: "Cascade All" seems to cause this issue. If Assign was set to "Cascade None" we were able to reassign the Opportunity with no problem.