Please, explain in details what you mean (Unauthorized error?)
Still, I fear that proxy roles in workflow scripts stopped working properly ages ago (at the time of Plone 2.5): you probably must switch to a simple scripts that call a custom view.
Stay away from external methods: they still works perfectly but are largely deprecated.
@erasmu maybe also check that when you assign Manager proxy role to the script in the ZMI that it works as you expect. Setting the role via the .metadata file is a separate issue maybe.