/libraries/civicrm/extern/soap.php

for unsubscribe links and other tools. On a default Drupal 8 install, such a URL is blocked by the .htaccess file. To fix that, add these lines:

  RewriteCond %{REQUEST_URI} !/libraries/civicrm/extern/authorizeIPN.php$
  RewriteCond %{REQUEST_URI} !/libraries/civicrm/extern/cxn.php$
  RewriteCond %{REQUEST_URI} !/libraries/civicrm/extern/ipn.php$
  RewriteCond %{REQUEST_URI} !/libraries/civicrm/extern/open.php$
  RewriteCond %{REQUEST_URI} !/libraries/civicrm/extern/pxIPN.php$
  RewriteCond %{REQUEST_URI} !/libraries/civicrm/extern/rest.php$
  RewriteCond %{REQUEST_URI} !/libraries/civicrm/extern/soap.php$
  RewriteCond %{REQUEST_URI} !/libraries/civicrm/extern/url.php$
  RewriteCond %{REQUEST_URI} !/libraries/civicrm/extern/widget.php$

to that file just after this line:

  RewriteCond %{REQUEST_URI} !/core/modules/statistics/statistics.php$

That should fix it.