Help please Fatal error: Call to undefined function get_current_blog_id()

I am trying to resolve an issue for my new client https://www.hswigcentre.ie. He is unable to log in to the Dashboard. He gets this Fatal error Fatal error: Call to undefined function get_current_blog_id() in /home/**/public_html/wp-includes/user.php on line 671. I have tried renaming the plugins folder but that didn’t work. I also tried renaming the theme so it would switch back to 2011 but to no avail.
Can anyone help please??