DIAGNOSIS The brew basket is clogged, either from too-fine coffee grounds or a dirty filter paper clogging the single hole at the bottom.
FIX Stop using paper filters if you are. The permanent metal mesh filter is designed for this machine. If you must use paper, ensure it's the correct #4 cone size and doesn't collapse into the hole. More critically, switch to a coarser grind. Fine grinds create a mud that blocks water flow. Clean the permanent filter thoroughly after every use with a brush under hot water.
IF THAT DIDN'T WORK The one-way valve in the basket hinge might be stuck open. Inspect the small silicone flap inside the basket's spout. Ensure it moves freely and isn't gummed up with oils. Clean it with a vinegar soak.
HEADS UP Never exceed the MAX line on the brew basket. Overfilling with grounds, even with a coarse grind, will cause backup and overflow. The basket capacity is smaller than the carafe capacity.