In case anyone else runs into this issue:
Using the ReadOnly user I'm creating further reports to help analyse the Tableau Server. For one of my views I'm using _background_tasks and decided to use the "link" field as it should give a relevant URL for object - It doesn't.
This link field is not set up correctly: it links to any object of the same name, but doesn't take into account the server that object is on. Background tasks are run on the server and not site-specific, but they are made site specific by the object that they refer to. Unfortunately that is not taken into account in the _background_tasks view.
I've logged this with the Tableau support but I'm very surprised by their answer: Apparently they believe the link is not viable by design.
Un-Tableau like design in my opinion but heyho .
In case you want to report on multiple sites and use _background_tasks: ignore the link and build your own.
It is not reliable at best (it might point to an object with the same name on a different site) and broken for the rest.
Update: Matt from TechSupport called to discuss this further and has decided it might need to be looked at as a defect after all.
Please keep us in the loop on what happens! This could be useful to document for other customers.
PS. I unmarked this post as a question because it seems like you're not looking for an answer. If I'm wrong let me know :-)