Users using Das 7.3.x or below are getting this error when attempting to change FQDN
If the customer uses Computer Logon, check the Agent Public URL...
Unfortunately, due to a bug, which has been fixed in version 8, the change FQDN does not recognise the TCP protocol; therefore, the workaround is either to change the protocol here to HTTP, or temporarily remove the URL and then change the FQDN.