Getting 521 error code on legal connection

Hello guys,

I always get the 521 error from Cloudflare while accessing my site on SSL.
I already tested the connection:
LISTEN 0 16 XXX.XX.XX.XXX:443* and it listens correctly.

Now the weird thing: When I visit the with a domain from another DNS provider, it works and I receive content.
Only the domain from Cloudflare is blocking the connection with a 521 error.

Can someone help me please?

Well, a 521 means exactly what it says in the article you already linked. So there’s not more to say on that.

In your case it might simply be a firewall which blocks Cloudflare connections. Check for that.

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