Dear Brian, thank you for response.
How can I check WHMCS logs? Where can I find them!?
This is the mail the support team sent me:
Thank you for contacting WHMCS Support, I hope your day is going well. This issue seems to be because of your WHMCS Bridge plugin. You can see the invoice and page will load fine if it is set to:
Please make sure that you have set your System URL to https://hogionline.com/billing/. I'd recommend contacting the developer of the plugin for further assistance.
My WHMCS settings are okay and everything is set as they told me, but it still can't work.
If I change the whmcs link to billing as suggested on the whmcs support ticket, the site isn't integrating my WordPress installation.
But If I remain with whmcs_bridge link the site integrates perfectly my WordPress installation, but I get the 404 page after order placement.
See attached pictures