Exception during "Differenzierte Synchronisation" #5

Twig\Error\RuntimeError:
An exception has been thrown during the rendering of a template ("Warning: Array to string conversion") in "@Contao_ContaoCoreBundle/be_main.html5".

  at vendor/contao/core-bundle/contao/templates/backend/be_main.html5:-1
  at Twig\Template->yield()
     (vendor/twig/twig/src/Environment.php(420) : eval()'d code:51)
  at __TwigTemplate_9fdebde094254ef541e2d529ba0ba618->doDisplay()
     (vendor/twig/twig/src/Template.php:402)
  at Twig\Template->yield()
     (vendor/twig/twig/src/Template.php:358)
  at Twig\Template->display()
     (vendor/twig/twig/src/Template.php:373)
  at Twig\Template->render()
     (vendor/twig/twig/src/TemplateWrapper.php:51)
  at Twig\TemplateWrapper->render()
     (vendor/twig/twig/src/Environment.php:333)
  at Twig\Environment->render()
     (vendor/symfony/framework-bundle/Controller/AbstractController.php:448)
  at Symfony\Bundle\FrameworkBundle\Controller\AbstractController->doRenderView()
     (vendor/symfony/framework-bundle/Controller/AbstractController.php:453)
  at Symfony\Bundle\FrameworkBundle\Controller\AbstractController->doRender()
     (vendor/symfony/framework-bundle/Controller/AbstractController.php:253)
  at Symfony\Bundle\FrameworkBundle\Controller\AbstractController->render()
     (vendor/contao/core-bundle/src/Controller/AbstractBackendController.php:51)
  at Contao\CoreBundle\Controller\AbstractBackendController->render()
     (vendor/menatwork/synccto/src/Controller/ClientController.php:468)
  at MenAtWork\SyncCto\Controller\ClientController->__invoke()
     (vendor/symfony/http-kernel/HttpKernel.php:181)
  at Symfony\Component\HttpKernel\HttpKernel->handleRaw()
     (vendor/symfony/http-kernel/HttpKernel.php:76)
  at Symfony\Component\HttpKernel\HttpKernel->handle()
     (vendor/symfony/http-kernel/Kernel.php:197)
  at Symfony\Component\HttpKernel\Kernel->handle()
     (public/index.php:42)