Login for author/editor not working (export.php)

Hello,
I’m using wordpress 4.3 and a Themler theme. I have the problem that when I login as an admin everything is ok and working, but if I login as an author or editor I get following error message:

Warning: Invalid argument supplied for foreach() in URL/wp-content/themes/WEBNAME/export/export.php on line 155

Warning: Cannot modify header information - headers already sent by (output started at URL/wp-content/themes/THEME/export/export.php:155) in URL/wp-includes/option.php on line 787

Warning: Cannot modify header information - headers already sent by (output started at URL/wp-content/themes/THEME/export/export.php:155) in URL/wp-includes/option.php on line 788

What I’ve done so far:
I deleted all my plugins = problem still there
I removed the function.php = problem solved
I checked for any whitespaced, empty line before or after the php opening and closing tag = there was none
I checked the export php, the code from line 155 (bold) is

function theme_add_export_option_page() {
    add_theme_page(__('Billion Themler', THEME_NS), __('Billion Themler', THEME_NS), 'edit_themes', 'theme_editor', 'theme_editor');
    global $submenu;
    <strong>foreach($submenu['themes.php'] as $key => $value) {</strong>
        if (in_array('theme_editor', $value)) {
            unset($submenu['themes.php'][$key]);
            break;
        }

I chnaged the Theme to the default theme and this solved the problem, too. So it is theme related, but I want to keep the theme and the admin login is working, so I hope this is something I can solve without getting rid of the theme.

Any ideas?
Thanks for your help