Can you post a screenshot of the error? Are you accessing AGOL over http or https? It could be a mixed content issue, as I don't see any problems when I use http to access AGOL and use the http service URL:
On the other hand, if I use https to access AGOL and http for the service URL, that introduces an mixed content issue, as you can't mix http and https:
Some browsers allow you to ignore mixed content issues, but if you're accessing the webmap through https, try using http instead. Unfortunately, if your organization is SSL only, that means you can only access the web map viewer over https, and the Server site hosting the service doesn't appear to have SSL enabled.