DIAGNOSIS Old, rancid coffee oils are baked onto the internal metal parts—primarily the stainless steel shower head and the brew basket. This taints every brew.
FIX You need a deep clean. Remove the shower head and brew basket. Soak both in a solution of hot water and 2 tablespoons of baking soda OR a dedicated coffee machine cleaner like Urnex for 30 minutes. Scrub with a soft brush. Rinse thoroughly. Then, run multiple water-only brew cycles. Finally, wipe down the entire brew chamber with a vinegar-dampened cloth.
IF THAT DIDN'T WORK Your water temperature might be too high. Try the 'Rich' setting instead of 'Classic' or 'Over Ice'. 'Rich' uses a slightly lower temperature to reduce bitterness extraction. Also, ensure you're not using too many grounds for the selected cup size.
HEADS UP Never leave wet grounds sitting in the basket. Clean the basket and shower head immediately after each use. Oils go rancid within hours and create that permanent burnt flavor.