Remote Trigger Always Returns 0

I have set up a remote-triggered action which worked for a while. It has now stopped.
To avoid any problems with my actions, I have re-created a simple macro:

remote trigger > display message

The problem is: My initial macro and all test macros I have created so far do not fire.
Accessing the macro via the browser or terminal/get requests return "0" which indicates a problem where the macro cannot be found on the server, right?

I am not behind a firewall (as far as I read, it should not impact the trigger anyway) and I am connected to the internet via a fixed IP adress. I can access normally.

How do I solve this?