DIAGNOSIS The water is getting too hot, scalding the coffee. This is almost always due to mineral scale buildup on the aluminum heating plate inside the unit, which insulates it and causes it to overheat to transfer heat.
FIX You need to descale the internal tank and heating plate. Run a full carafe of white vinegar through a brew cycle (no coffee). Let it sit in the carafe for 15 minutes, then run it through again with just fresh water for two full cycles to rinse. This dissolves the scale directly on the heating surface.
IF THAT DIDN'T WORK Check if you're using the correct grind. A too-fine grind for a drip machine will over-extract and taste bitter. Use a medium grind.
HEADS UP Never use harsh descaling chemicals like CLR; they can damage internal seals. Stick to vinegar or citric acid.