Error each time I save changes

Each time I update a page, the update works but I get the following warming messages:

“Warning: strtolower() expects parameter 1 to be string, array given in /home4/name/public_html/manchesterpostbasicdysphagiacourse.com/wp-includes/formatting.php on line 1887

Warning: Cannot modify header information – headers already sent by (output started at /home4/name/public_html/manchesterpostbasicdysphagiacourse.com/wp-includes/formatting.php:1887) in /home4/name/public_html/manchesterpostbasicdysphagiacourse.com/wp-includes/pluggable.php on line 1179”

Can anyone tell me how to fix this?