Reverse Proxy and embedding HTTP400 Bad request

I have a reverse proxy setup via IIS to hook metabase up to a domain.
It's all working perfectly, apart from signed embedding. Public embedding works but the preview when setting up signed embedding throws a "400 Bad request"

Here's a SS of my metabase general settings tab:

And my troubleshooting info:

  "browser-info": {
    "language": "en-US",
    "platform": "Win32",
    "userAgent": "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:109.0) Gecko/20100101 Firefox/111.0",
    "vendor": ""
  "system-info": {
    "file.encoding": "Cp1252",
    "": "OpenJDK Runtime Environment",
    "java.runtime.version": "17.0.6+10",
    "java.vendor": "Eclipse Adoptium",
    "java.vendor.url": "",
    "java.version": "17.0.6",
    "": "OpenJDK 64-Bit Server VM",
    "java.vm.version": "17.0.6+10",
    "": "Windows Server 2019",
    "os.version": "10.0",
    "user.language": "en",
    "user.timezone": "Europe/Berlin"
  "metabase-info": {
    "databases": [
    "hosting-env": "unknown",
    "application-database": "mysql",
    "application-database-details": {
      "database": {
        "name": "MariaDB",
        "version": "10.11.2-MariaDB"
      "jdbc-driver": {
        "name": "MariaDB Connector/J",
        "version": "2.7.6"
    "run-mode": "prod",
    "version": {
      "date": "2023-03-28",
      "tag": "v0.46.0",
      "branch": "release-x.46.x",
      "hash": "0ca7df3"
    "settings": {
      "report-timezone": null

Very strange since everything else runs perfectly fine through the proxy.

Edit: It seems to work properly in production. Just the preview that is not working. Why this is I do not know but I can live with it.

that's weird, the endpoint that gets called when doing a preview it's called preview_embed:

Strange. I have not tested it since 0.46.x but will update this once I do