If You are running WHMCS v. 5 or v.6 and using cPanel server module (for creating cPanel hosting accounts), then in August cPanel removed XML API from stable WHM branch (WHM v. 74) which means cPanel module stopped working reporting errors:
An unknown error occurred
Module log shows:
<!DOCTYPE html>
<html lang="en" dir="ltr">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta name="google" content="notranslate" />
<meta name="apple-itunes-app" content="app-id=1188352635" />
...
to fix it for for WHMCS 5 and 6 and allow cpanel accounts creation/suspension/termination, try to backup /modules/servers/cpanel/cpanel.php and instead add cpanel.php from attached .zip archive, it should work.
last update. 30.11.2018 (password change functionality update)