Sorry about this! I really want to create a stable platform that people can build upon without having annoying problems, so it's top priority for me to solve stuff like this. It's hard enough to debug custom code without having to deal with stuff like this 😓

That said, would you be able to share a character URL? I tried this code and it worked correctly:

oc.thread.on("MessageAdded", async function() {
  let result = await fetch("https://httpbin.org/post", {
    method: "POST",
    headers: { "Content-Type":"application/json" },
    body: JSON.stringify({foo:3})
  }).then(r => r.json());
  console.log("POST response:", result);
});

If it works for you now, then maybe it was a temporary problem. If that's the case, I'll set up a monitor to do this once per minute, and see if I can catch any problems and correlate them with server logs/errors.