If you have installed the latest firmware version 10.5 on your XMS server you probably notice that when you try to connect using the FQDN it’s not working anymore even if the certificate is correct.
This change is due to a Security Fix: “Citrix XenMobile Server HTTP Host Header Injection Vulnerability”
So now, you can only connect using the IP address or the FQDN, it’s written on Citrix web site 😉
Note: Those information are provided based on my own experience.