Our current site (web.miniextensions.com) does not use an API key at all since the connection is established in an entirely different manner (OAuth).
Users of our legacy site (app.miniextensions.com) need to switch to the Personal Access Token system as explained in this article.
Most likely the issue here is that the entered API key is incorrect. Go back to the app settings by clicking on the gear icon in the top-right corner (or the dropdown menu). Then copy/paste the miniExtensions API key again.
