When you have an MP11x device with V4 software that needs to be upgraded to V6, the upgrade has to be performed in 2 steps.
First the upgrade needs to be done to V5. After the device is running in V5 the upgrade to V6 can be performed.
In case you tried to upgrade the system in one step, the fail LED (red) will burn.
The device can still be rescued by using the BootP application. With this tool the system can be upgraded again to the desired software.
The bootp programm and a description can also be found here.