I am told by the site maintainer that upgrading PHP and installing the multibyte string extension fixed it! I just tested it out and can confirm that it worked for me. Here was the error (which is no longer happening):
PHP Fatal error: Uncaught Error: Call to undefined function mb_ereg_replace() in /var/www/setsideb.com/public_html/wp-content/plugins/anthologize/includes/class-tei-dom.php:1047\nStack trace:\n#0 /var/www/setsideb.com/public_html/wp-content/plugins/anthologize/templates/epub/includes/class-epub-builder.php(22): TeiDom->getFileName()\n#1 /var/www/setsideb.com/public_html/wp-content/plugins/anthologize/templates/epub/index.php(29): EpubBuilder->__construct()\n#2 /var/www/setsideb.com/public_html/wp-includes/template.php(783): require_once('...')\n#3 /var/www/setsideb.com/public_html/wp-content/plugins/anthologize/includes/class-admin-main.php(373): load_template()\n#4 /var/www/setsideb.com/public_html/wp-includes/class-wp-hook.php(308): Anthologize_Admin_Main->load_template()\n#5 /var/www/setsideb.com/public_html/wp-includes/class-wp-hook.php(332): WP_Hook->apply_filters()\n#6 /var/www/setsideb.com/public_html/wp-includes/plugin.php(517): WP_Hook->do_action()\n#7 /var/www/setsideb.com/public_html/wp-admin/includes/menu.php(155): do_action()\n#8 /var/www/setsideb.com/public_html/wp-admin/menu.php(428): require_once('...')\n#9 /var/www/setsideb.com/public_html/wp-admin/admin.php(158): require('...')\n#10 {main}\n thrown in /var/www/setsideb.com/public_html/wp-content/plugins/anthologize/includes/class-tei-dom.php on line 1047, referer: https://setsideb.com/wp-admin/admin.php?page=anthologize_export_project&project_id=2734