[Risolto] Multiple domain setup - image src broken on non-English sites

Background of the issue:
I am trying to see images on a translated site. The issue can be seen on this page: link nascosto. I expected to see featured images, but instead, I encountered a problem with the image source URLs.

Symptoms:
In the console, I see de2.hdbuzz.net/de2.hdbuzz.net/files being fetched because the src for featured images starts with src="de2.hdbuzz.net/files..." when it should start with src="/files or src="link nascosto. When I examine attachment URLs in PHP, they are returned like this when they need a slash or https:// at the start.

Questions:
Why are the image src URLs incorrect on the translated site?
How can I fix the image src URLs to display correctly?