When I bought mine – virtually the same engine (2020D) there was brown sludge in the header tank. I doubt that anything is wrong. You just need to drain the old antifreeze, flush the system well then refill. Use the ‘proper’ Volvo green stuff (from Keypart) and dilute with distilled water (Halfords) not tap water. It is supposed to be changed every 2 years and move has remained clean now that I do that. Getting the drain plug out is a real chafe. It is definitely worth getting a 10mm drain plug socket (eBay) and a set of extension bars. Then you get a good grip of the plug and your tools don’t get tangled with the throttle cable.