A 50% decline in output in 3 years is not normal, assuming your weather has been similar in both comparison months. It indicates there is new panel shading, or perhaps one out of 2 panel strings has no output, maybe due to an isolator problem or corroded connection. Shading of one panel can wipe out almost the entire string's output in sunny weather, but in overcast weather doesn't have such a large effect, due to the diffuse light source and lack of dense shadowing (assuming the shadow is not due to something very close to the panel).
What size system is it? How many strings of panels? The inverter should show inputs from each string by scrolling through the menu, and you may be able to determine where the problem is- you need to check several times throughout the day for shadowing issues. Alternatively, if you have decent system monitoring and logging, the problem may be much simpler to pick up.