Wow! Thanks for translating!
I think the light indicates a coolant-related problem, possibly low coolant level. I suppose, through normal driving, the coolant is circulating sufficiently enough to prevent the coolant sensor from detecting a problem.
I would check the coolant expansion tank first to see if there is enough coolant in the tank. If so, then perhaps the sensor is defective...
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car