We've updated the source code of our RSEvents! 2Checkout Plugin, due to several changes that the payment processor has made lately regarding the "Instant Notification Service".
The update doesn't affect the plugin configuration (the 2Checkout Id, secret word, routine and mode are still required in the Joomla! backend panel, "Plugin Manager" area) and it only addresses to the 2Checkout vendor account.
Additionally, 2Checkout requires to enter an "Instant Notification URL" in your 2Checkout account, similar to the above example:
For example, if your website domain is http://www.sell-clothes.com, you will need to enter the following URL:
More information about the 2Checkout "Instant notification URL" can be found here.