The "Error:Flow Removed" and "Error:Flow Not At Task" errors errors occur when you open an assignment and the assignment doesn't match any page on the work object's pxFlow page. This usually means you have a data integrity issue. Either a stale version of the work object or the assignment was written to the database, overwriting the correct version. This usually happens when there is a manual commit added (or an inappropriate Obj-Save with Write Now selected). I would start by looking at the work object history of the items in this state to see if there is a common path through the flow that gets them into this state. Then I'd trace that and look for commits or saves in your custom code.