To reproduce fire this in your browser console (not here on the forums though):
fetch("https://online-go.com/api/v1/players?username=runarberg").then(d => d.json()).then(d => console.log(d))
Here on the forums and on the site we get the expected response, but on another site (e.g. http://localhost or https://avavt.github.io/gotstats) we get a CORS error (but only on Firefox).