Can't Login – wrong parameter count and cannot modify header information errors

Can someone please offer me some help with an issue that just occurred on my website.

When I try to log in I get the following errors…

Warning: Wrong parameter count for session_destroy() in /homepages/2/d272873542/htdocs/wordpress/wp-includes/plugin.php on line 406

Warning: Cannot modify header information – headers already sent by (output started at /homepages/2/d272873542/htdocs/wordpress/wp-includes/plugin.php:406) in /homepages/2/d272873542/htdocs/wordpress/wp-includes/pluggable.php on line 876

Does anyone have any ideas on the cause and/or solution please? Web site is running normally for visitors…
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