Unable to read response, or response is empty error

Hi! I am using Magento 2.2.7 on a dedicated VPS server (CentOS 7).

I am trying to active intregration with Klaviyo service.

When I click on System -> Integration -> Activate I get this error in var/log/exception.log:

[2022-04-01 14:12:37] main.CRITICAL: Unable to read response, or response is empty {"exception":"[object] (Zend_Http_Client_Exception(code: 0): Unable to read response, or response is empty at /home/domain/public_html/vendor/magento/zendframework1/library/Zend/Http/Client.php:1093)"} [] 

I also checked with tcpdump and it seems there is some issue with remote CA:

https://imgur.com/kAXr5jI

I followed this guide (since Klaviyo is hosted behind CF network):

https://developers.cloudflare.com/cloudflare-one/connections/connect-devices/warp/install-cloudflare-cert/

Anyway, still no luck.

Any suggestion?

Thank you very much!