Fatal Error

Impossible at the moment to use the contact form. In the log, I can see a fatal error. Could you please help me:

PHP Fatal error: Uncaught Error: Call to undefined function wp_get_list_item_separator() in /app/wordpress/wp-content/plugins/contact-form-7/includes/mail.php:389
Stack trace:

#0 [internal function]: WPCF7_MailTaggedText->replace_tags_callback(Array)

#1 /app/wordpress/wp-content/plugins/contact-form-7/includes/mail.php(349): preg_replace_callback(‘/(\[?)\[[\t ]*(…’, Array, ‘[_site_title] “…’)

#2 /app/wordpress/wp-content/plugins/contact-form-7/includes/mail.php(266): WPCF7_MailTaggedText->replace_tags()

#3 /app/wordpress/wp-content/plugins/contact-form-7/includes/mail.php(207): wpcf7_mail_replace_tags(Array, Array)

#4 /app/wordpress/wp-content/plugins/contact-form-7/includes/mail.php(56): WPCF7_Mail->replace_tags(‘[_site_title] “…’, Array)

#5 /app/wordpress/wp-content/plugins/contact-form-7/includes/mail.php(99): WPCF7_Mail->get(‘[_site_title] “…’, true)

#6 /app/wordpress/wp-content/plugins/contact-form-7/includes/mail.php(19): WPCF7_Mail->compose()

#7 /app/wordpress/wp-content/plugins/contact-form-7/includes/submi in /app/wordpress/wp-content/plugins/contact-form-7/includes/mail.php on line 389

The page I need help with: [log in to see the link]