InvalidArgumentException after upgrade to beta7

After upgrading to beta7, the testsite displays an error

nvalidArgumentException: The URI "node/add" is invalid. You must use a valid URI scheme. in Drupal\Core\Url::fromUri() (line 302 of core/lib/Drupal/Core/Url.php).

Drupal\link\Plugin\Field\FieldType\LinkItem->getUrl()
Drupal\shortcut\Entity\Shortcut->getUrl()
shortcut_preprocess_page(Array, 'page', Array)
Drupal\Core\Theme\ThemeManager->theme('page', Array)
Drupal\Core\Theme\ThemeManager->render('page', Array)
Drupal\Core\Render\Renderer->doRender(Array, 1)
Drupal\Core\Render\Renderer->render(Array, 1)
Drupal\Core\Render\MainContent\HtmlRenderer->renderResponse(Array, Object, Object)
Drupal\Core\EventSubscriber\MainContentViewSubscriber->onViewRenderArray(Object, 'kernel.view', Object)
Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher->dispatch('kernel.view', Object)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\PageCache->pass(Object, 1, 1)
Drupal\Core\StackMiddleware\PageCache->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1)
Stack\StackedHttpKernel->handle(Object, 1, 1)
Drupal\Core\DrupalKernel->handle(Object)