Content Type Gallery: Unexpected Error for document template

I have also struggled with this; anyone know of any specific documentation on how to correctly enter the URL and the site requirements for the source document library?

I know there are limitations with regards to where the source document library is. If I recall correctly it has to be in the same site or subsite of where the content type is created, meaning if I create the content type on a subsite, I can't use a link to a source document in a document library that is on another branch of the root site.

In theory there is a way to do this by using the Content Type Gallery so you can link to a source template file in any document library regardless of site structure, but I still fail on providing a valid link.

There is another thread on this topic, but it does not have a valid solution.
Error when using an URL for document template in Content Type | Microsoft Community Hub