Error – please please help.

every time I try to click on opdate after editing a page I get a blank screen. Tried to deactivate plugins.
Is it a wordpress error or a host error? And how do I fix it?

De_bug gives me this:

Notice: Den benyttede constructor-metode for WP_Widget er forældet siden version 4.3.0! Benyt

__construct()

i stedet. in /customers/6/7/9/safiren.dk/httpd.www/wp-includes/functions.php on line 3624
Notice: Den benyttede constructor-metode for WP_Widget er forældet siden version 4.3.0! Benyt

__construct()

i stedet. in /customers/6/7/9/safiren.dk/httpd.www/wp-includes/functions.php on line 3624
Warning: Cannot modify header information – headers already sent by
(output started at /customers/6/7/9/safiren.dk/httpd.www/wp-content/themes/christina/content/widgets-importer.php:2)
in /customers/6/7/9/safiren.dk/httpd.www/wp-content/plugins/wordpress-mobile-pack/inc/class-wmp-cookie.php on line 50
Warning: Cannot modify header information – headers already sent by
(output started at /customers/6/7/9/safiren.dk/httpd.www/wp-content/themes/christina/content/widgets-importer.php:2)
in /customers/6/7/9/safiren.dk/httpd.www/wp-admin/post.php on line 197 Warning: Cannot modify header information – headers already sent by
(output started at /customers/6/7/9/safiren.dk/httpd.www/wp-content/themes/christina/content/widgets-importer.php:2)
in /customers/6/7/9/safiren.dk/httpd.www/wp-includes/pluggable.php on line 1228