I have been getting a number of these 521 error notifications for a few weeks now. The errors at first were on pages that I have permanently/indefinitely cached in the Cloudflare cache settings. My origin server has been up for over a year without issues.
But now I am receiving 521 error notifications from a Cloudflare Page site, so their own servers are returning errors.
Cloudflare clearly have some bug in their origin scanner, but there is no way to inform them of this because unfortunately their support line has shut down.