InvalidArgumentException: The user-entered string '' must begin with a '/', '?', or '#'

Just double checked that after removing the default domain, I can access the /admin/content page again. Now, the main page (www.example.com) is not accessible.

I ran into this with a misconfigured Site Studio component and a Link field that was returning "" because it could not look up the user input value, it did not resolve to any entity in the database.

The problem was here:

  /**
   * {@inheritdoc}
   */
  public function processOutbound($path, &$options = [], Request $request = NULL, BubbleableMetadata $bubbleable_metadata = NULL) {
    // Load the active domain if not set.
    $active_domain = $options['active_domain'] ?? $this->getActiveDomain();

    // Only act on valid internal paths and when a domain loads.
    $external = $options['external'] ?? FALSE;
    if (is_null($active_domain) || $path === '' || $external === TRUE) {
      return $path;
    }

    // Set the default source information.
    $source = NULL;
    $options['active_domain'] = $active_domain;
    $entity = NULL;

    // Get the current language.
    $langcode = NULL;
    if (isset($options['language'])) {
      $langcode = $options['language']->getId();
    }

    // Get the URL object for this request.
    $alias = $this->aliasManager->getPathByAlias($path, $langcode);
    $url = Url::fromUserInput($alias, $options);

I think at a minimum the lines:

    // Get the URL object for this request.
    $alias = $this->aliasManager->getPathByAlias($path, $langcode);
    $url = Url::fromUserInput($alias, $options);

should be wrapped in a try catch. Log the path, request, options, alias and return $path. That gives someone a chance to trace back where the input came from, potentially, and lets pages load.

I could not access the Content admin, URL Alias page, URL Redirect page, or the Admin Content view or areas like that until I resolved this.

To get back up and going to debug the problem for your specific local database, you can do:

if ($path == '') {
  return $path;
}

    // Get the URL object for this request.
    $alias = $this->aliasManager->getPathByAlias($path, $langcode);
    $url = Url::fromUserInput($alias, $options);

in DomainSourcePathProcessor.php. That will prevent the pages from crashing so you can trace back the error. In my case, I snagged a database, fixed the content issue, and pushed the database back under maintenance to get the site back online.

I tried looking at other examples of outbound processors in core and contrib, but I could not locate any that themselves called Url::fromUserInput. The only route I can think of is to at least try/catch here so it prevents the OP from occurring and offers some way of recovering and fixing the data.

This was really hard to replicate - somehow my issue (and assume OP issue) was that $path was passed as NULL (or ''). Which should not happen, and the how it happened... not quite sure. But a try/catch won't harm anything here.