1. Check if both Site / Machine ID (MID) codes are really valid (correct).

As these are hexadecimal values they can contain only numbers (0-9) and letters (A-F). Instruct end user to copy/paste codes or automate this process from your application by using advanced protection interface. Using custom buttons with appropriate action string for sending codes via email is also an option. 

2. If Site / Machine ID (MID) codes are generated by application protected with old PC Guard for Win32/.NET V5 be sure to check V5 option in activation panel.

3. If not, be sure that you are using correct project settings for generating activation codes. It is possible that protected application was protected with different project settings.