Drupal 8 sub-directory installation – redirect error

So I’ve successfully managed to install Drupal 8 in a sub-directory. The problem is I haven’t figured out how to correctly redirect /user to /sub-directory/user. Currently I get the following error:
Redirects to external URLs are not allowed by default, use DrupalCoreRoutingTrustedRedirectResponse for it.

There were several other topics that I found on this that recommended adjusting the trusted_host_patterns in settings.php, but I have already done that (prior to noticing the error).

I’m pretty good at theming Drupal, but I don’t even know where to start with this. Help?

Drupal version: 

Source: https://www.drupal.org/taxonomy/term/1/feed