From the contest:
"This can happen if a security check is failed, or in the case that the first 128 bits of the SHA-1 of the newly created encryption key don‘t match on both parties’ clients when this stage is completed (this corresponds to Paul and Nick comparing the key visualizations for the Secret Chat in their Telegram apps)."
https://konklone.com/post/why-google-is-hurrying-the-web-to-...
From the contest: "This can happen if a security check is failed, or in the case that the first 128 bits of the SHA-1 of the newly created encryption key don‘t match on both parties’ clients when this stage is completed (this corresponds to Paul and Nick comparing the key visualizations for the Secret Chat in their Telegram apps)."