Hi , whatever activity was happening right before you saw the 503 error is what caused this one. Likely a newly installed extension or upgrade.
As the stack trace shows, there is a fatal error because the function name must be a string in /homepages/41/d606839977/htdocs/kuul/app/code/core/Mage/Core/Model/Layout.php.
That file along with the activity performed just before the 503 error should help you track down the issue and solution.