According to your description, everything seems to be fine with your Oskar. Of course, the red LED comes on again when you lift the lid (lifting the lid = not ready for operation). So if there is enough water in it, the Oskar is ON and it still doesn't start running, it can only mean that the humidity in the room is already high enough. (50-55% is recommended as optimal. You can achieve this with level 4 (55%) in the hygrostat setting). At level 5 it runs in continuous operation. Respectively, it should now - Otherwise, the manufacturer's website may be helpful ;)

The appliance only runs with the lid on and at least a minimum water level. The red LED indicates that the water level is too low. As long as it is lit, the unit cannot be put into operation. If Oskar is filled and lights up red with the lid on, I would suspect a defect.