can’t authenticate

That’s a fairly generic WordPress error message. Are you able to look at your server’s error logs?

Yes. What should I be looking for?

Fatal errors generated at the same time that you see that error message.

Here are the server errors we found:
[Sat Aug 31 17:06:14.824249 2019] [php7:error] [pid 1383] [client 172.16.1.48:28972] PHP Fatal error: Uncaught Error: Class ‘SimpleXMLElement’ not found in /var/www/html/simplenexus.com/sn/wp-content/plugins/pardot/includes/pardot-api-class.php:451\nStack trace:\n#0 /var/www/html/simplenexus.com/sn/wp-content/plugins/pardot/includes/pardot-api-class.php(128): Pardot_API->get_response(‘login’, Array, ‘api_key’)\n#1 /var/www/html/simplenexus.com/sn/wp-content/plugins/pardot/includes/pardot-settings-class.php(546): Pardot_API->authenticate(Array)\n#2 /var/www/html/simplenexus.com/sn/wp-content/plugins/pardot/includes/pardot-settings-class.php(409): Pardot_Settings->authenticate(Array)\n#3 /var/www/html/simplenexus.com/sn/wp-includes/class-wp-hook.php(288): Pardot_Settings->sanitize_fields(Array)\n#4 /var/www/html/simplenexus.com/sn/wp-includes/plugin.php(208): WP_Hook->apply_filters(Array, Array)\n#5 /var/www/html/simplenexus.com/sn/wp-includes/formatting.php(4720): apply_filters(‘sanitize_option…’, Array, ‘pardot_settings’, Array)\n#6 /var/www/html/simplenexus.com/sn/wp-includes/option.php(306): sanitize_option(‘pard in /var/www/html/simplenexus.com/sn/wp-content/plugins/pardot/includes/pardot-api-class.php on line 451, referer: https://simplenexus.com/sn/wp-admin/options-general.php?page=pardot

Got it. Are you able to enable simplexml on your server? This is usually on by default.

I’ll add this to the list of things we need to improve from the old code base. It shouldn’t assume that module is active.