Maybe it's just the log which is screwing up the characters and the real problem is with the inability to resolve your hostname. It doesn't make any sense to use your external domain name and port to switch a local switch on the same system.
You should use
There is no security issue as you are directly piping the command to localhost and it doesn't leave the system.
Furthermore should you add 127.0.0.1 to the accepted networks in domoticz settings.
I really can't imagine
https://mijn.fqdn.nl doesn't need authentication, so there's your second problem. You let Domoticz execute the command to open that URL, it makes an request to your domain which routes it back to your IP and shows up as remote access to Domoticz. It can't successfully do that because there is no username and password supplied.
If you have the possibility, login to your NAS with SSH and execute
I expect something like
'HTTP request sent, awaiting response... 401 Unauthorized
Authorization failed.'