Premature end of script headers: index.php

If I enable realname and set a token triugh the admin interface I get the message

Premature end of script headers: index.php

in my apache error log and the site went down with an 500 Internal server error.

I am on Debian 7

An d I use php5 as fcgi Module

wit apache2 mpm-prefork

Drupal works grat on ths vserver but realnem does not. There was a similar error some yeares ago. This one was closed, because it could not be reproduced