OpenRefine on wikibase.cloud: Can't define statemens in schema "something went wrong"

I’ve created a hosted wikibase using wikibase.cloud. I’ve also set up OpenRefine using the tutorial by @lauraneckstein as recommended in this thread. I am able to connect to my wikibase instance and reconcile data, e.g. add columns from reconcield values. Reconciliation service is up and running under http://localhost:8000/en/api.

However, adding statements in the schema is not possible. The error message is: “Sorry, something went wrong. Please try again later.”

Console log issues this warning: “Cross-origin request blocked”, although I’ve no idea what this means and if it’s in any way related to my problem. I’de be grateful for any hints or suggestions.