How to fix "Failure - Unknown" while using Speculative Rules in Google Chrome?

Question

Using javascript I insert next script

<script type="speculativerules">
  {
    "prerender":[{
      "source":"list",
      "urls":["/cart"]
    }]
  }
</script>

My server correctly responds with the cart page and 200 HTTP status. However I see an error "Failure - Unknown" in chrome devtools. I see no way how I can investigate this trouble further and make Speculation Rules work.

One thing I noticed: sometimes it helps to toggle Preload Pages tumbler in browser's settings.