Child theme produces empty pages and empty media library

Hi
we have a very strange error.
Although our actual childtheme contains nothing than a correct CSS and a almost empty functions.php we often get get blank pages and errors

This is the content of our css:

/*
 Theme Name:     Foto-Theme
 Description:    Fotostudio Lauers Enfold Child Theme
 Author:         Fotostudio Lauer
 Author URI:     http://www.........
 Template:       enfold
 Version:        0.0.1
*/

@import url("../enfold/style.css");

/* =Theme customization starts here
-------------------------------------------------------------- */

This is the content oof our functions.php:
<?php ?>

We stripped the content of our functions.php down to the state yxou see above just to check if the php file did contain any syntax-errors.

memory limit available: 96 MB
memory needed (checked) : around 45 MB

Debug:On
Errors:Warning: Cannot modify header information – headers already sent by (output started at /usr/www/users/axella/wordpress/wp-content/themes/enfold-child/functions.php:1) in /usr/www/users/axella/wordpress/wp-content/themes/enfold/framework/php/function-set-avia-backend.php on line 484

Help would be very appreciated.
regards axel