Error 400 with Zero Trust cloudflare tunnel

I am using Zero Trust Tunnel to deploy an https website (http on the local side).

I get a lot of 400 errors in the browser log:
Uncaught (in promise) Error: Request failed with status code 400.

There are no entries or errors visible in the logs of the web server. It seems that Cloudflare is not forwarding these requests to the origin server.

When I click several times, the website sometimes works, but I still get error 400.

It goes the Application ScriptRunner (
They gave us the following tips on what it could be:

[#1] - restriction in the free tier / restrictions on concurrent requests - this URL here would

[#2] - Response header:
The response header differs from each other, so maybe it doesn’t match with the call anymore.

If we connect to the website directly, everything works fine without any errors.
We tried different tunnel settings, but we can’t fix the problem.
Any ideas where this could come from and if it’s solveable?