Error in whmcs: "No matching admin user found" but user and wordpress gets created & D


I get an error when I want to accept an order.

"No matching admin user found"

But the user and his website are created in my wordpress multisite install.
How can I fix this? In an other install it works fine, the only difference I can see is that the new domain uses ssl.

Another question I have is about assigning domains to the user's website. How does this work exactly?
Do I need to install the domain mapping plugin? or is this handled by the WHMCS MU Provisioning?
It is kind of confusing in your description. What I want to achieve is when a client orders his website and a domain that it gets mapped automatically. I can't seem to figure out how this can be achieved.

Thanks in advance!!
Ilyas Deckers