If you are unable to connect to your customer using LogMeIn Rescue, please ask the customer to hold down the Ctrl + Alt + Shift  keys before clicking the Connect to Technician button.  This method forces the session into a relayed connection and resolves issues in peer to peer connections such as NAT not working properly on both ends.

If the connection continues to fail or an Invalid certificate message appears, there may be a security policy on the customer's network preventing the connection.  UDP communication and ActiveX must be allowed for Rescue connections to take place.