It seems that you're using an outdated browser. Some things may not work as they should (or don't work at all).
We suggest you upgrade newer and better browser like: Chrome, Firefox, Internet Explorer or Opera

When I save my game, I cannot overwrite previous saves. It gives me the warning about overwriting saved games, and I click OK and give it a name, but when I go back to the loading screen, the previous save has not been overwritten and my progress has been lost. This applies to normal save slots and the autosave.
This question / problem has been solved by Senseiweaselimage
If you have an antivirus, it could be running the game in sandbox mode or other limited function.