DIAGNOSIS The three small rubber feet on the bottom of the carafe have probably worn down, gotten hard, or one has fallen off. These feet are critical for a level, stable seal on the warming plate.
FIX Flip the carafe over. Inspect the three small, clear rubber bumpers. If one is missing, you'll need a replacement. If they're just hardened, you can sometimes soften them by soaking in very warm water, but they likely need replacing. Mueller sells spare parts, including these feet.
IF THAT DIDN'T WORK The lip of the warming plate itself could be warped from heat over time, but that's far less common than the carafe feet wearing out.
HEADS UP Don't put the hot carafe directly on a cold, hard counter—that thermal shock can crack it. Use a trivet. Also, losing that seal means the keep-warm function is way less effective because heat is escaping.