Given new state is NULL

Setup, Configuration and Use Scripts & Rules
1

From Error to Error …

Error Message in Log

[runtime.busevents             ] - ardu02_20_temp_raw_onboard state updated to 3992
[s.r.ardu02_20_temp_raw_onboard] - 3992
[runtime.busevents             ] - ardu02_20_temp_onboard_lastupdate state updated to 2017-02-14T13:48:59
[.c.i.events.EventPublisherImpl] - given new state is NULL, couldn't post update for 'ardu02_20_temp_onboard'

Rules

rule "rule_ardu02_20_temp_raw_onboard_update"
when
	Item ardu02_20_temp_raw_onboard received update
then
    postUpdate(ardu02_20_temp_onboard_lastupdate, new DateTimeType())
    logInfo("rules.ardu02_20_temp_raw_onboard", (ardu02_20_temp_raw_onboard.state as Number).intValue.toString)

    try
    {
    	postUpdate(ardu02_20_temp_onboard, String::format("%1$.2f", ((ardu02_20_temp_raw_onboard.state as Number).intValue * 0.0078125)))
    }
	catch (Throwable t)
	{
    	postUpdate(ardu02_20_temp_onboard, -99)
	}
end

Items

Number	ardu02_20_temp_raw_onboard		"RAW OnBoard Temperatur 0x20 [%d]"		<temperature>	(grp_homebb01_portexpander_ardu02_raw_temp, grp_homebb01_portexpander_raw_temp)									{ mqtt="<[mosquitto_cubie:ardu02/pub/1w/temp/28cb4a8e03000005:state:default]" }
Number	ardu02_20_temp_onboard			"OnBoard Temperatur 0x20 [%.2f °C]"		<temperature>	(grp_homebb01_portexpander_ardu02_temp, grp_homebb01_portexpander_temp, grp_homebb01_persistence_mysql)

Number	ardu02_20_scan_onboard_raw_temp	"RAW Scan Temperatur [%d]"				<temperature>	(grp_homebb01_portexpander_ardu02_raw_temp, grp_homebb01_portexpander_raw_temp)									{ mqtt="<[mosquitto_cubie:ardu02/pub/1w/scan/28cb4a8e03000005/temp:state:default]" }
Number	ardu02_20_scan_onboard_temp		"Temperatur [%.2f °C]"					<temperature>	(grp_homebb01_portexpander_ardu02_temp, grp_homebb01_portexpander_temp, grp_homebb01_persistence_mysql)

DateTime	ardu02_20_temp_onboard_lastupdate			"Letztes Update [%1$td.%1$tm.%1$ty %1$tH:%1$tM:%1$tS]"	<clock>
DateTime	ardu02_20_scan_onboard_lastupdate			"Letztes Update [%1$td.%1$tm.%1$ty %1$tH:%1$tM:%1$tS]"	<clock>
2

So, break down the complicated expression in your postUpdate to a step by step process, and see where it goes wrong

A regular tip in these forums is to use item.postUpdate(x) method instead of postUpdate(item, x) action - it is more likely to convert the ‘x’ to something useful

3 
postUpdate(ardu02_20_temp_onboard, (ardu02_20_temp_raw_onboard.state as DecimalType) * 0.0078125)

that is working now.