Error notifications/warnings when trying to get onto wordpress site

I recently upgraded to wordpress 3.6, and since, I’m not able to access the wordpress admin page. When I try to login (tried different browsers) I get the following bunch of errors:

Warning: call_user_func_array() expects parameter 1 to be a valid callback, function ‘_canonical_charset’ not found or invalid function name in /hosts/aegee-maastricht.org/wordpress/wp-includes/plugin.php on line 173

Warning: call_user_func_array() expects parameter 1 to be a valid callback, function ‘_canonical_charset’ not found or invalid function name in /hosts/aegee-maastricht.org/wordpress/wp-includes/plugin.php on line 173

Warning: Cannot modify header information – headers already sent by (output started at /home/bhofabi06.de/hosts/aegee-maastricht.org/wordpress/wp-includes/plugin.php:173) in /hosts/aegee-maastricht.org/wordpress/wp-login.php on line 384

Warning: Cannot modify header information – headers already sent by (output started at /home/bhofabi06.de/hosts/aegee-maastricht.org/wordpress/wp-includes/plugin.php:173) in /hosts/aegee-maastricht.org/wordpress/wp-login.php on line 396

Warning: Cannot modify header information – headers already sent by (output started at /home/bhofabi06.de/hosts/aegee-maastricht.org/wordpress/wp-includes/plugin.php:173) in /hosts/aegee-maastricht.org/wordpress/wp-includes/pluggable.php on line 680

Warning: Cannot modify header information – headers already sent by (output started at /home/bhofabi06.de/hosts/aegee-maastricht.org/wordpress/wp-includes/plugin.php:173) in /hosts/aegee-maastricht.org/wordpress/wp-includes/pluggable.php on line 681

Warning: Cannot modify header information – headers already sent by (output started at /home/bhofabi06.de/hosts/aegee-maastricht.org/wordpress/wp-includes/plugin.php:173) in /hosts/aegee-maastricht.org/wordpress/wp-includes/pluggable.php on line 682

Warning: Cannot modify header information – headers already sent by (output started at /home/bhofabi06.de/hosts/aegee-maastricht.org/wordpress/wp-includes/plugin.php:173) in /hosts/aegee-maastricht.org/wordpress/wp-includes/pluggable.php on line 876

When I go to the site as a ‘visitor’ I see the following on the top of the site:

Warning: call_user_func_array() expects parameter 1 to be a valid callback, function ‘_canonical_charset’ not found or invalid function name in /hosts/aegee-maastricht.org/wordpress/wp-includes/plugin.php on line 173

Warning: call_user_func_array() expects parameter 1 to be a valid callback, function ‘_canonical_charset’ not found or invalid function name in /hosts/aegee-maastricht.org/wordpress/wp-includes/plugin.php on line 173

And when I try to go directly to the plugins page, or to edit a page, I get the following:

Fatal error: Call to undefined function wp_unslash() in /hosts/aegee-maastricht.org/wordpress/wp-admin/admin.php on line 46

It is important that this get solved asap, as next week there’s an event taking place for which there needs to be info on the site, which I currently cannot get on it.

I’m not an IT specialist, not by far, so simple explanations would be best, thanks!