Hello @SylvainW 

Good day!!

I think, Databricks does not allow localhost as an embedding domain. ( may be other can confirm this)

work around for you: 

1. Use a local tunnel / reverse proxy - Then add that temporary hostname (e.g.,https://ngrok(or)localtunnel ) to your Databricks Approved domains list. 

2. Use a dev subdomain- use dev.yourcomany.com in Databricks and whitelist

3. Skip iframe and use API calls - For testing rather than UI, you can fetch the dashboard data via the Databricks SQL/API and render it directly. (complex)

(open for other solutions)