Symptoms

Error is thrown when moving a workflow to a new category
 

Diagnoses

When moving a workflow to a new category an error is thrown: Index was out of range. Must be non-negative and less than the size of the collection. Parameter name: index.

 

Resolution

This is a known issue.

When moving multiple items including the workflow, the error does not encounter. Seems only to be an issue when moving a single workflow.

As a workaround try to move the process with another item.