The issue of the “Place Order” resulting in showing “{“result”:”success”…” is not an issue with Woocommerce. It is an issue with one of the plugins that you have installed.
This happens when there is some code (HTML or Javascript) which is being printed by the conflicting plugin after the order is placed. I had faced a similar problem with my plugin a week ago & the Javascript code that I had added was being printed after the “Place Order” button is clicked. This code though, is not visible on the page. You have to look into your Firefox/chrome console.
For developers: the fix is to make sure that the HTML/Javascript is displayed only when the $_POST array is blank or it’s count is 0.
For non-developers: the fix is to identify the plugin causing this, contact the developer & have them fix it or disable that plugin.
You can also drop an email to [email protected] if you want me to have a look at it (for free).