question

Are you in the Northern hemisphere, if so solar yield drops off significantly from September to December, see a typical UK chart below.a

Check your connections and fusing. The history showing battery voltage dropping to 0 the past couple of days strongly indicates that you have a loose connection or degraded fuse between the controller and the battery bank or at the battery bank itself. Most likely not a blown fuse, as if that was the case there wouldn't be any charging possible, but a loose connection or degraded fuse would explain both the 0v reading at night and, too, would explain some of your charging woes.

What type of batterie are you running? LiFePO4 with "wet" profile #2, a thermo-switch might cut off to 0-volt, or if you use flooded ones, is there enough water in them, are they frozen at night?

Remember: cloudy, shady days mean you might get only 10-20% of your normal solar watts.Yesterdays 880Wh / (2*400W*15%) = 7 to 8h muddy "sunshine". Assuming 2pc 12V 100Ah (= ~2,4kWh) refilled with 0,88kWh is not that much, and assuming old fridge 60W ... (24h*0,06kW* 33% duty cycle means 0,475kWh cooled away)

Any relay - with cut-off function - connected to virtual load output?

Any left food bags near batterie/solar cables ,which attracted martens, weasels, squirrel?

Check out the suggestions re cabling etc. However assuming no cable faults this is normal in winter except for the zero volt readings. 600W of solar on my van won't keep up with my fridge and trickle charge to the starter battery.