DIAGNOSIS The one-way valve in the thermal carafe lid is stuck shut or the lid isn't fully locked. This creates a vacuum, so brewed coffee can't flow into the carafe properly and backs up.
FIX Disassemble and clean the carafe lid thoroughly. Twist the black silicone gasket out of the lid. Remove the white plastic one-way valve piece from the center. Wash all parts in warm soapy water, rinse, and let dry. Reassemble, ensuring the valve piece clicks into place and the silicone gasket is seated evenly.
IF THAT DIDN'T WORK The carafe itself might be the issue. Ensure you are using ONLY the official Ninja thermal carafe designed for this model. The gurgling and overflow happen with generic or older Ninja carafes because the seal height is wrong.
HEADS UP Never brew directly into the thermal carafe without the lid properly attached and locked (turn it until it clicks). Brewing into an open carafe can cause immediate overflow and damage the brewer's internal sensors.