Tableau Prep Builder error code: System error: Unable to publish the datasource. Verify the project exists on your server and you have permission to access it, or select a different project.

I am getting the above error code when publishing output in one of my flows.

I am the site creator and administrator and checked every permission on project-workbook-datasources-.. and nothing amiss.

I can run every other flow perfectly, but not a single output in this particular one.

What do I check next?