Cloudflare (CF) suddenly sends 521 error email? But the site is normal.

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Cloudflare (CF) suddenly sent an email warning about a 521 error? However, the site is normal

[There was] an increase in the number of 521 errors on your source service.
•*.com
[Please check this article from our help center:
•Error 521: The web server has been closed
[The 521 error occurs when your source web server refuses to connect to Cloudflare's connection]
•Two common causes for 521 errors:
•Your source web server application is offline
•Cloudflare requests are blocked
[If you need more background information about this alert, please refer to this community tip. For guidance on how to solve this problem, please seek advice and insights on the forum or contact Cloudflare support]
]