historicState returns a HistoricItem. This has both the timestamp of the state and the state itself. You need to use
CurrentHum_LaCrosse_Arbeitszimmer.historicState(now.minusMinutes(5)).state as Number
I recommend Number of DecimalType. You do not need to call floatValue to compare it to 5.
Consider using a Timer instead of a long sleep like this. Long sleeps can cause problems.