Nsurl error domain (1012) subscribing MDM Server
The problem is solved. The problem was caused by the certificates.
I really needed signed certificates for "mdmserver.domainname.be" (Server > Certificates).

But keep in mind, it has to be with a CSR created by the os server, it's not enough to create a CSR with a CSR generator (like directadmin, ..). Thats because you can't add your own private key/csr, the server has its own.

More information about the certificates install: https://www.digicert.com/ssl-certificate-installation-mac-osx-mavericks.htm
SSL Installation Diagnostics Tool: https://www.digicert.com/help/