@gabi92a, I found out why Fiddler wasn't working. For NodeJS apps like the TRIGGERcmd agent, you have to set these environment variables before you run the node src\agent.js --console command:

set https_proxy=http://127.0.0.1:8888 set http_proxy=http://127.0.0.1:8888 set NODE_TLS_REJECT_UNAUTHORIZED=0

That tells the NodeJS app to use Fiddler as a proxy server.
I found that here: https://weblogs.asp.net/dixin/use-fiddler-with-node-js

Can you try that? Maybe that will give us a clue.

Also, you could try running it after setting this variable to get a bunch of debug output:

set NODE_DEBUG=*

One thing you should know - the agent uses a websocket to connect to the server, and that connection doesn't use the https_proxy variable so Fiddler won't pick that up. The request that's giving us this error doesn't use websockets though, so that shouldn't matter for this troubleshooting.