Today the 2.4 version is released. The mixed content fixer has been rebuilt, which should make it work in almost all situations. The marker which makes it possible for the plugin to detect if it’s working has been changed so that it won’t conflict with ajax calls anymore.
In some setups, the admin notices do not show up, due to plugins that hide these notices. I don’t think that’s a good thing: plugin developers should use notices with caution, but they should be visible. But as this made it impossible for these users to enable Really Simple SSL, a button to enable SSL is added on the settings page.
The Really Simple SSL pro 1.0.19 will be released tomorrow, and will include: the HSTS preload list option, mixed content fixer for the back-end, and HSTS for multisite. The new features of 1.0.19 will only work as of 2.4.0