"Redirects to external URLs are not allowed by default" in detection configuration by domain

One of our clients encountered this last week. Their setup also involves different domains for different languages.

I could reproduce it locally by using en.localhost, fi.localhost, dk.localhost, etc.

So I went into the code with Xdebug. What happens is that when you delete a translated node, it redirects you to the node's default translation. In a setup with different domains this means that it tries to redirect you to another domain, which is not a "SecuredRedirectResponse". This check is in \Drupal\Core\EventSubscriber\RedirectResponseSubscriber::checkRedirectUrl

The node is deleted just fine, just that the redirect triggers an error.

I made sure that it's reproducible using only core. It's reproducible with the standard profile installation.

Steps to reproduce:

1. Enable modules: Language and Content translation
2. Configure a second language, eg. Swedish
3. Configure Language > Detection and selection > URL to use Domain based, and add domains for each language
4. Configure Content language and translation so that Basic page is translatable
5. Create a node in English
6. Translate the node to the second language
7. Delete the second language's translation
8. Encounter error: "Redirects to external URLs are not allowed by default, use \Drupal\Core\Routing\TrustedRedirectResponse for it."

Adding related issue I finally found after searching some more.