DIAGNOSIS The bucket float or the sensor that detects it is stuck or misaligned. There's a small plastic float inside the bucket that rises with the water level, and a lever/switch in the unit that gets pressed when the bucket is seated properly.
FIX Take the bucket out again. Inspect the float inside the bucket—make sure it moves up and down freely in its chamber. Rinse it out, as sludge can jam it. Then, look inside the unit where the bucket slides in. You'll see a small plastic lever or pin. Manually press it a few times to make sure it's not stuck in the "pressed" position. Wipe any debris from that area. Reinsert the bucket firmly until it clicks.
IF THAT DIDN'T WORK The micro-switch that the lever activates might have failed. This requires opening the cabinet to access and test the switch. If it's bad, you'd need to solder in a new micro-switch, which is a more advanced repair.
HEADS UP A lot of people bypass this by just running the unit with a continuous drain hose, which is fine. But if you ever want to use the bucket again, you'll need to fix this, otherwise the unit will falsely think it's full and shut off after two minutes every time.