DIAGNOSIS This is the blender's built-in overload protection kicking in. It's not necessarily about liquid volume—it's sensing the motor is straining, often because the contents are too thick or frozen solid at the bottom.
FIX First, stop and unplug it. You'll need to manually break up the vortex. 1) Take the pitcher off the base. 2) Use a long spoon or spatula to stir and loosen everything at the bottom, especially any frozen fruit or nut butter. 3) Add a bit more liquid (even just a quarter cup of water or your base liquid) to help things move. 4) Reassemble and try a lower speed (like 1 or 2) to start, then ramp up.
IF THAT DIDN'T WORK The sensor itself might be overly sensitive or faulty. Try a known-good, easy blend: just two cups of water on a medium speed. If it still flashes, there's likely an issue with the base's sensor or motherboard. Contact Ninja support at that point.
HEADS UP This model really doesn't like trying to blend ultra-thick stuff (like nut butters or frozen bananas alone) from a dead stop. Always start with liquid in first, then softer ingredients, and put the hardest, frozen items on top so they get pulled down into the blades.