XS2 (SMB1) and XS41 (SMB2) have the same connections have the same pinout. Loss of battery backup fault may be due to data mismatch or you could have a weak SMU battery.
IRB120 brake connections are handled thru the MP1 connector.
When trying to move the robot, brakes will not release unless contactor K44 engages.
Since it happens when trying to move and thru the brake release button, double-check connections going to the contactor board and G1 power supply.