Getting a 521 Error - What's Causing It and How Do I Fix It? or Why Am I Seeing a 521 Error and How C

Cloudflare’s 521 error is so vague, right?

For me, it was mod_security blocking legit traffic. Try disabling it temporarily:

`sudo nano /etc/apache2/mods-enabled/security.conf`

Comment out the mod_security line and restart Apache.

If it works, tweak your rules instead of disabling it completely.