I have the same problem as this https://premium.wpmudev.org/forums/topic/whmcs-integration-stop-on-configuration-step
I only have WHMCS Integration Plugin http://www.3alienswebhosting.com/ When I place an order I get to the part of domain enter use my own and input the domain and tdl and hit enter it sits there Grayed out and does nothing.
I try to go to my other sites and the server is locked up. I have to reboot Apache to get the server back.
If I go to the Client by its self with out Wordpress all is well. This is a stock WHMCS with the Portal theme.
BLOGS MU theme
I just changed the cart to another one is Works fine so there's something with the modern cart thats not working.
If you look at my setup and don't see Wordpress I'm looking at WHMCS themes to try them out. Weighing my options.