DIAGNOSIS That's a leak from the main seal. Coffee-ground sludge is escaping because the seal between the top and bottom chambers has failed, usually because the pot wasn't screwed together tightly enough, or the gasket is damaged.
FIX 1) Immediately turn the switch to 'O' and unplug the base. Let it cool. 2) Unscrew the pot (carefully, it's hot) and check the large black silicone gasket. Clean it and inspect for any tears, nicks, or if it feels flattened and brittle. 3) Check the mating surfaces on both aluminum chambers for any coffee grounds or debris—a single ground can break the seal. 4) Reassemble, ensuring the gasket is perfectly seated in its groove, and screw the two halves together as tightly as you can by hand.
IF THAT DIDN'T WORK The threading on the aluminum might be crossed or damaged. Make sure you're screwing it on straight and not forcing it if it feels gritty or stuck. A damaged thread will never seal properly.
HEADS UP Always wipe the rim of the lower chamber and the gasket area bone-dry before assembling. Even a few drops of water can act as a lubricant and prevent you from getting a tight, leak-proof seal.