can you give a serial number from truck dataplate?

on trucks with the AC2 controller: Alarm on Mod 5 is for hydraulic pump circuit
VMN Low indicates low voltage on the motor circuit
when you connected a higher voltage battery it just went High because it saw a higher voltage than expected.

according to the manual it says to check the pump motor and circuits
Possible causes :
a) There is a problem in the motor connections or in the motor power
circuits; check that the 3 phases are connected correctly;
check if there is loss to earth from the motor;
b) Fault in the inverter - replace the controller

I would check cables, fuses and make sure the UAV connections on the controller are correct.
I would pay close attention to the motor because it does have a sensor bearing in it, those have been known to cause problems when they go bad.
This sensor bearing is also sometimes called an encoder bearing, i did find in another section of the troubleshooting manual about the pump encoder troubleshooting and it describes the encoder error as an 05 alarm error.

It sounds like the encoder bearing in the pump motor may have failed triggering that fault code alarm.