Error 521 occur all of the sudden

Hi my alicloud ecs is working fine with cloudflare the last few weeks.
And it show error 521 all of the sudden today.

As it worked fine previously so I do not think that it is something to do with configuration.

This is what I get now. I can access to my page when i keep refreshing but it dead when i refresh again.

Please help

A 521 error happens when Cloudflare is unable to make a TCP connection to your origin server. Review the suggestions in this Community Tip for Quick Fix Ideas.

i have whitelisted all the ip and allow the port that cloudflare used.
Any idea on how to check it whether is cloudflare problem or server problem?
note that it run nicely previous.

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