after upgrading to Safari 9 I'm getting this error in the browser:
[Warning] [blocked] The page at https://localhost:8443/login was not allowed to run insecure content from http://localhost:8080/assets/static/script.js.
Anyone knows how to enable the running of insecure content on the new Safari?
According to the Apple support forums Safari does not allow you to disable the block on mixed content.
Though this is frustrating for usability in legitimate cases like yours, it seems to be part of their effort to force secure content serving / content serving best practices.
As a solution for you you can either upgrade the HTTP connection to HTTPS (which it seems you have done) or proxy your content through an HTTPS connection with an HTTPS-enabled service (or, in your case, port).