Cloudflare 521 only when Imunify360 is active (origin works fine)

Michel

• April 09, 2026 09:14
• Edited

Hello,

we are experiencing an issue on multiple cPanel servers with Imunify360 + Cloudflare proxy.

When Cloudflare proxy is enabled, websites return Error 521 (Web server is down).
However, the origin server is fully functional.

Key point

Everything works normally if:

• Cloudflare proxy is disabled (DNS only), or
• Imunify360 is disabled

What we verified

• server and services are running normally
• ports 80/443 are open and listening
• direct access to origin IP works
• access via hosts file works
• local requests (curl localhost) return valid responses

What we tried

• verified correct DNS (A record → origin IP)
• whitelisted all Cloudflare IPv4 and IPv6 ranges
• added Cloudflare as trusted proxy (remote-proxy) in Imunify360
• reloaded Imunify360 configuration

Result

No change — the issue only occurs when both Cloudflare proxy and Imunify360 are active.

Has anyone seen similar behavior recently?
Could this be related to a recent Imunify360 rule/update?

Thanks.

• 

  cPRex Jurassic Moderator

  • April 09, 2026 13:53

  Hey there!  I reached out to the Imunify team and there is an ongoing issue right now they've identified:

  https://imunify360.statuspage.io/incidents/810xhk4nbqnr

  I'd recommend using the "subscribe to updates" button there so you get a notification once this is fully resolved, since it doesn't sound like restarting the service is changing the behavior for you.

  1
• 

  Michel

  • April 09, 2026 21:29

  Hello! 
  
  Thanks a lot! I was trying to find if there was already something reported regarding this issue, but it seems I completely missed it. Appreciate you pointing it out — thanks again!

  0
• 

  cPRex Jurassic Moderator

  • April 09, 2026 23:36

  You're very welcome!

  0
• 

  sibony88

  • April 14, 2026 05:50

  Hello , i had the same problem today 521 on all of my websites. only after i disabled the proxy on all domain and clicked the developer mode the sites start running again

  0
• 

  sibony88

  • April 14, 2026 07:47

  is ther any updates ?

  0
• 

  Michel

  • April 14, 2026 08:41

  Hello,
  
  Unfortunately, it seems there is no permanent fix yet, only a temporary workaround.

  We’ve tested it on our side and after applying it, the site is loading normally again.

  Workaround: Disable the WebShield service:

  imunify360-agent config update '{"WEBSHIELD": {"enable": false}}'

  They’ve confirmed they are still working on a permanent fix.

  More information can be found on the link that cPRex provided.

  Hope this helps!

  1

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