DIAGNOSIS The machine thinks the filter hasn't been reset after cleaning. There's a separate reset step you have to do.
FIX First, make sure the filter is completely dry and clicked all the way back into its slot. With the unit powered on, press and hold the Filter Reset button on the control panel for about 3 seconds. You should hear a beep and the red filter light should turn off. If it doesn't, try holding it for up to 5 seconds.
IF THAT DIDN'T WORK Unplug the dehumidifier for a full minute to do a hard reset. Plug it back in and immediately press and hold the Filter Reset button again. Sometimes the control board just needs a fresh start.
HEADS UP This model counts run hours, not dirt. You need to press that reset every 250 hours of operation, even if you haven't cleaned the filter, or the light will come on and the unit may reduce its performance.