Problem with contact forms (FormIt) after upgrade to 3.0.0 - Solved

Summary

I’ve upgrade my site from 2.8.3 to 3.0.0-pl today. The site and the manager seem to be working well, extras such as simple search and others working. But none of my contact forms work. After filling them out and hitting submit I’m presented with a blank page in the browser.

Modx Error log shows:
[2022-04-05 07:48:08] (ERROR @ /var/www/core/vendor/xpdo/xpdo/src/xPDO/xPDO.php : 666) Could not load class: modRestClient from rest.modrestclient
[2022-04-05 07:48:08] (ERROR @ /var/www/core/vendor/xpdo/xpdo/src/xPDO/xPDO.php : 1267) Problem getting service rest, instance of class rest.modRestClient, from path

Environment

Modx 3.0.0-pl (Advanced)
Formit 4.2.7-pl
Apache 2.4
PHP 7.2.24
Mysql 5.7.37

Still not exactly sure what caused the issue in the first place. But Some forms are working. Trying to fix the rest of them now.

Newer forms work fine, but older ones don’t. Something in my older Formit code must be causing issue.