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.