MPE+ can't connect, and iTunes reports the device is "supervised with another computer and cannot be used with this computer." What does this mean, and is there a way around this?

This is caused by an app called Apple Configurator, which is often used to mass configure and administer iOS devices. This app allows the administrator to configure a "Supervisor" computer, preventing the device from communicating with any other computer. MPE+ can bypass this "lock" by pointing it to the pairing record obtained from the "Supervisor" computer.