I've tried contacting Mixcloud about this but they don't appear to support the FootWidget api as consider it to be experimental. I'm trying to incorporate this into a WP site and have managed to get it working in Firefox but cannot fathom why it's not working in Chrome & Safari. I've checked the console output, Query Monitor and PHP error logs but can't figure out the disconnect. I get the following errors in Chrome:

Failed to execute 'postMessage' on 'DOMWindow': The target origin provided ('') does not match the recipient window's origin ('')

Followed by:

player-widget_vendor.31ac4e7571f324849be7m.js:19 SyntaxError: Unexpected end of JSON input at player-widget.5ffe47295603dad997b3m.js:5

Then when I click an item to load into the player I get:

player-widget_vendor.31ac4e7571f324849be7m.js:18 POST 403

Any suggestions welcome.