I’m getting the same issue, glad there is a solution here 
Weird thing is that it seems to only be affecting rebuilds done today. Other servers (same Discourse version - Stable 2.7.7) are running ok.
Though started getting some strange errors recently on a few forums (not after an update):
Oops
The software powering this discussion forum encountered an unexpected problem. We apologize for the inconvenience.
Detailed information about the error was logged, and an automatic notification generated. We'll take a look at it.
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Not sure if it’s related, the error was why I was recently doing rebuilds on the same version.
Edit: Confirmed, removing the Cloudflare template fixed the 521 error.
Update: Looks like @Zenexer made a PR for this: Cloudflare template broken again