I have a few webforms on my D8 site. They all work for anonymous site visitors except for the contact form, which gives an "access denied" error.

The permissions under the form's Settings->Access tab are the same as for other forms that do work.

The form's access permissions are set to allow anonymous / authenticated / admin users to create and view/ update own submissions:

I did search for the solution but what I found didn't help. I tried the debugging code from https://drupal.stackexchange.com/a/315766 and get the following messages:

The website encountered an unexpected error. Please try again later.

Exception: ACCESS DENIED: in *Symfony\Component\HttpKernel\Exception\AccessDeniedHttpException->__construct() (*line 28 of /path/to/vendor/symfony/http-kernel/Exception/AccessDeniedHttpException.php).

    Drupal\Core\Routing\AccessAwareRouter->checkAccess(Object) (Line: 94)
    Drupal\Core\Routing\AccessAwareRouter->matchRequest(Object) (Line: 113)
    Symfony\Component\HttpKernel\EventListener\RouterListener->onKernelRequest(Object, 'kernel.request', Object)
    call_user_func(Array, Object, 'kernel.request', Object) (Line: 111)
    Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher->dispatch('kernel.request', Object) (Line: 127)
    Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 68)
    Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 57)
    Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 47)
    Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 191)
    Drupal\page_cache\StackMiddleware\PageCache->fetch(Object, 1, 1) (Line: 128)
    Drupal\page_cache\StackMiddleware\PageCache->lookup(Object, 1, 1) (Line: 82)
    Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 47)
    Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 52)
    Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
    Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 708)
    Drupal\Core\DrupalKernel->handle(Object) (Line: 19)

I don't know what this all means. Where do I go from here? Thank you.