下記のようなメッセージになりました。

Whoops, looks like something went wrong.
(1/1) PDOException

SQLSTATE[HY000]: General error: 2006 MySQL server has gone away
in Connection.php line 1278
at PDO->beginTransaction()in Connection.php line 1278
at Connection->beginTransaction()in Connection.php line 1384
at Connection->rollBack()in EntityManager.php line 262
at EntityManager->rollback()in EntityManager_9a5be93.php line 100
at EntityManager_9a5be93->rollback()in TransactionListener.php line 107
at TransactionListener->onKernelException(object(GetResponseForExceptionEvent), 'kernel.exception', object(TraceableEventDispatcher))in WrappedListener.php line 104
at WrappedListener->__invoke(object(GetResponseForExceptionEvent), 'kernel.exception', object(ContainerAwareEventDispatcher))in EventDispatcher.php line 212
at EventDispatcher->doDispatch(array(object(WrappedListener), object(WrappedListener), object(WrappedListener), object(WrappedListener), object(WrappedListener), object(WrappedListener), object(WrappedListener)), 'kernel.exception', object(GetResponseForExceptionEvent))in EventDispatcher.php line 44
at EventDispatcher->dispatch('kernel.exception', object(GetResponseForExceptionEvent))in TraceableEventDispatcher.php line 139
at TraceableEventDispatcher->dispatch('kernel.exception', object(GetResponseForExceptionEvent))in HttpKernel.php line 228
at HttpKernel->handleException(object(Twig_Error_Runtime), object(Request), 1)in HttpKernel.php line 79
at HttpKernel->handle(object(Request), 1, true)in Kernel.php line 200
at Kernel->handle(object(Request))in index.php line 58