Cron tasks don't work: Could the Domain Access module be the cause?
The error is coming obviously from these lines of code in the cron settings form, when it tries to build the cron link:

$cron_url = $this-&gt;url('system.cron', ['key' =&gt; $this-&gt;state-&gt;get('system.cron_key')], ['absolute' =&gt; TRUE]);
$form['cron_url'] = [
  '#markup' =&gt; '&lt;p&gt;' . t('To run cron from outside the site, go to &lt;a href=":cron"&gt;@cron&lt;/a&gt;', [':cron' =&gt; $cron_url, '@cron' =&gt; $cron_url]) . '&lt;/p&gt;',
];


So you somehow lost the State variable system.cron_key, which is on your site "" and causes the error. You find this in the database in the table key_value and collection state.

To set a new key you can rerun this code from system_install():

  // Populate the cron key state variable.
  $cron_key = \Drupal\Component\Utility\Crypt::randomBytesBase64(55);
  \Drupal::state()-&gt;set('system.cron_key', $cron_key);