The website’s server didn’t answer. Not you.
Cloudflare tried to reach the website’s server and got no reply at all within its time limit. Overloads clear in minutes. A firewall or hosting problem lasts until the owner fixes it — often hours.
Nothing on your side causes this. VPN, cookies and browser do not matter here.
Allow Cloudflare’s IP ranges through the origin firewall and make sure the origin accepts connections on ports 80/443 from them; verify the DNS record points at the right server. Turn keep-alive on and check the server is not hitting its connection limits.
Cloudflare’s documentation