With a form set to captcha only I get "The website encountered an unexpected error. Please try again later." when trying to display the comment form.
The log shows:
LogicException: Form state caching on GET requests is not allowed. in Drupal\Core\Form\FormState->setCached() (line 494 of /home/ross/devel/d8test/www/core/lib/Drupal/Core/Form/FormState.php).
I may not understand the full implications but I guess you shouldn't cache a GET request containing a form because of the CSRF token among other things.
I solved this in Element/Mollom.php by removing this at line 298:
I'm happy to create but I though I'd see if there are any more correct solutions first.