Not sure on what could be causing this. Have you tried enabling the unsecured content on the browser? I would also request you try using the Enhanced Experience Composer.
I would also suggest you to try the steps Steven have mentioned above.
We had the same issue. The user performing the administration of the new Activity does not have the appropriate permissions. When you add the correct Groups you should be good to go.
[img]Screen Shot 2015-07-09 at 12.57.45 PM.png[/img]
Not sure on what could be causing this. Have you tried enabling the unsecured content on the browser? I would also request you try using the Enhanced Experience Composer.
I would also suggest you to try the steps Steven have mentioned above.