Data Links - Portal permissions issue - “You do not have permissions to access this resource or perform this operation."

I have a functioning server-side workflow which I'm using to form a 'Data Link' with some data in my project.

It only works when the sharing permissions on Portal are set to "Public", any other option doesn't work ('Owner' or 'Organisation').

“You do not have permissions to access this resource or perform this operation."

Any idea why this is happening?