After a lot of hours testing and reading I found the solution and the rule works again 
rule "Set daily max and min temperature für Außen"
when
Item Outside_Garten_Temperature changed or
Time cron "0 0 0 * * ?" or
System started
then
var Number Min_value = -99
var Number Max_value = -99
var String tmp
val time_formatter = java.time.format.DateTimeFormatter.ofPattern("HH:mm")
if (Outside_Garten_Temperature.state instanceof DecimalType) {
Min_value = (Outside_Garten_Temperature.minimumSince(now.with(LocalTime.of(0,0,0,0)), "influxdb").state as DecimalType)
var Min_time = Outside_Garten_Temperature.minimumSince(now.with(LocalTime.of(0,0,0,0)), "influxdb").timestamp
tmp = (Math::round(Min_value.floatValue*10.0)/10.0).toString() + "°C (" + Min_time.format(time_formatter) + " Uhr)"
postUpdate(Outside_Garten_Temperature_Min, tmp)
Max_value = (Outside_Garten_Temperature.maximumSince(now.with(LocalTime.of(0,0,0,0)), "influxdb").state as DecimalType)
var Max_time = Outside_Garten_Temperature.maximumSince(now.with(LocalTime.of(0,0,0,0)), "influxdb").timestamp
tmp = (Math::round(Max_value.floatValue*10.0)/10.0).toString() + "°C (" + Max_time.format(time_formatter) + " Uhr)"
postUpdate(Outside_Garten_Temperature_Max, tmp)
}
end