Cloudflare has implemented a fix and the issue should be resolved. All services are back up.

Cloudflare has identified the issue and is working on a fix. Read more on cloudflarestatus.com

Cloudflare is experiencing a massive global degradation affecting all proxied services and causing frequent errors.
It is unknown when the services will recover. Visit cloudflarestatus.com to get frequent updates on the issue.

Due to currently unknown reasons, services might return an internal server error / 500 error from time to time