Can't find recaptcha in Drupal 8
This might seem like a Drupal 8 issue, but it's actually a lurking core issue.
On line 81 of CRM/Utils/ReCAPTCHA.php, there's a require_once 'packages/recaptcha/recaptchalib.php'
The 'packages/' part of the path is unnecessary and breaks Drupal 8 sites because of the different path layout.
Probably worth doing a file scan to see if there are similar require_once issues.