We tested a theory that this was related to ElasticIP earlier today. And that does seem to be the cause, we were able to replicate this problem by starting an EC2 instance, then associating an ElasticIP, then trying to enable Cacti. The cacti graphs were broken images.
The solution was same as yours: restart Wowza service, after refreshing a few times the broken images were graphs and within a few minutes showed activity data.
We are updating the Wowza EC2 user guide with this information.
Richard