Amine

I see corrosion on those battery poles, my first guess would be continuous overcharge of the battery that caused hydrogen venting. Maybe start by checking the float voltage charge with a multi-meter. Maybe the charger is outputting the an incorrect voltage.
Next guess a battery that is failing and pulling the bank down causing the bulk charge to run an already fully charged battery.
Sorry my experience with FLA is limited, read up on them, but using them proved to be a pain in the butt so I moved on to LiFeSO4

I use the contact NC & C to switch a relay . Program 1 is set Sol . Our setup is offgrid . I use program 12 set 50 v to switch the relay off and 13 set to 51v to switch relay on . If you use the NO & C contact you can set 12 to switch on the light and 13 to switch off . Settings 12 and 13 can only adjust in full volts . You can also use program 1 Utility and set on low DC warning .

I use the 12 v because it also power some cooling fans . Just keep it within recommended range .

@Amine you must connect a power supply , the dry contact is just a switch . I use a 12 v power supply and a 1A fuse .

It's a pair of contacts; there is no power behind it. You have to provide power externally. NC stands for Normally Closed. Try your multimeter on the resistance (Ω) range.

Just like my setup; just a few months older than mine.
Yes. You could go to 72.70 factory firmware, or 73.00e patched firmware.
Instructions here.
There is. But you have two, right? 😉
More seriously, there is always a small risk, and whether it's worth the slight risk depends on what you want from the update. Do you use the LC Display a lot? Want to tinker with dynamic charge currents? Are you suffering from the premature float bugs?