When attempting a transaction, and the ITNs seem not to be going through to your application, go to https://developers.payfast.co.za/docs#ports-ips and unblock / whitelist the IPs and ports listed.
Should the issue persist, please check your cPGuard Web Application Firewall (WAF) and disable the following two options on the WAF integration settings:
RBL Protection
WEBSHELL Protection

Once this has been disabled, please attempt your transaction again, your ITNs should be going through.
The reason for the aforementioned workaround is due to the cPanel update on their cPGuard application having been set to view incoming ITNs from WooCommerce (& Joomla) as brute force phishing attacks. This is due to a pattern coming from Payfast for your ITN.
As this is the case, disabling the pattern blocker on your cPGuard will allow ITNs to push through and thus complete your transaction.
Please note: This is a workaround and we are endeavouring to find a complete solution for this issue.