Rebuild error

Hi, I get the following error anytime I hit the link to “Rebuild” when configuring a list – either in realtime or background:

A shutdown was triggered, possibly a fatal error was caught. Please note the message below when reporting this.

Array ( [type] => 1 [message] => Call to undefined function auser_multisort() [file] => /mnt/stor09-wc1-ord1/784656/954863/www.vcee.org/web/content/wp-content/plugins/amr-users/includes/ameta-building.php [line] => 641 )

Do you have any idea why this might be? It works ok if I hit the Rebuild Cache link, but not the Rebuild or Rebuild List links.

https://wordpress.org/plugins/amr-users/