This thread is resolved. Here is a description of the problem and solution.
Problem: Invalid site key for the current site.
Solution: - Back up your site first before making any change, this is for security reasons. - Go to Plugins -> Add new -> Commercial tab -> Click on 'Register WPML' button -> Uncheck 'Keep wpml.org up-to-date about which theme and plugins I use' option -> Insert the key and register the plugin.
This is the technical support forum for WPML - the multilingual WordPress plugin.