By default, Cloudflare is not API friendly including Uptime Kuma. Cloudflare may block requests from Uptime Kuma.
You need to disable "Browser Integrity Check" in Cloudflare Dashboard.
Related discussion: https://community.cloudflare.com/t/api-403-after-enabling-cloudflare/108078/6
Please read: https://github.com/louislam/uptime-kuma/wiki/Reverse-Proxy#cloudflare