White screen for blog, when adding own Google Fonts

Hi There,

We are having a white screen on the page we selected being the Blog, when we are using the following code to add our own Google Webfont in Child Theme:

add_theme_support('avia_template_builder_custom_css');

add_filter( 'avf_google_heading_font', 'avia_add_heading_font');
function avia_add_heading_font($fonts)
{
$fonts['Source Sans Pro'] = 'Source+Sans+Pro:400,300,700';
return $fonts;
}

add_filter( 'avf_google_content_font', 'avia_add_content_font');
function avia_add_content_font($fonts)
{
$fonts['Source Sans Pro'] = 'Source+Sans+Pro:400,300,700';
return $fonts;
}

The website is part of a multisite.

What can we do?

many Thanks, T.