On the same network as the OpenSprinkler, navigate to it’s web page without using any SSL or proxies. Goto: http://opensprinkler/su.
This will open a basic white page with two input boxes. The top is the Javascript URL which defaults to http://ui.opensprinkler.com and the other is the password which should be MD5 hashed.
Basically, the browser does not want to load non-secure content, is my understanding of the error.
Update: I think I fixed another issue regarding the loader and SSL issues. If you still have issues use https://betaui.opensprinkler.com as the Javascript URL on the /su page.