Numbers in Group name not allowed anymore?

Is anyone else seeing this behavior:

The following group is empty when I iterate through it:
Group:Switch gWeather1LightsOFF despite having members associated
when my rule wants to iterate through members of this group, it throws an error: cannot invoke method public java.util.Set org.eclipse.smarthome.core.items.GroupItem.getMembers() on null
(no other errors in log)

when I replace the number “1” with text “One” in the group name all works well;
this definition works:
Group:Switch gWeatherOneLightsOFF

The docs explicitly allow the use of numbers (together with letters and underscores)

Running on Ubuntu 16.04, all updated
Build #1189
java: 1.8.0_151-b12

Here the rule (adapted from Rick’s Time of Day DP )

rule "WeatherOfDay changed"
        Item WeatherOfDay received command
        Thread::sleep(100) // give lastUpdate time to catch up
        // Turn off previous Weather lights
        val lastTod = PreviousWeatherOfDay.state.toString
        val offGroupName = "g"+lastTod+"LightsOFF"
        logInfo("Lights", "Light levels turning off " + offGroupName)
        val GroupItem offGroup = gTimerLights.members.filter[ g | == offGroupName].head as GroupItem 
            offGroup.members.forEach[light | 
            logInfo("Lights", "Light levels turning OFF " + 
//        	Thread::sleep(100)
        // Turn on current Weather lights
        val onGroupName = "g"+receivedCommand+"LightsON"
        logInfo("Lights", "Light levels turning on " + onGroupName)
        val GroupItem onGroup = gTimerLights.members.filter[g |  == onGroupName].head as GroupItem
        onGroup.members.forEach[light |
                logInfo("Lights", "Light levels turning ON " +
//                Thread::sleep(100)

here one of the item definitions:
Switch Office_Lamp "Office Lamp" (gWeatherOneLightsON, gWeatherOneLightsOFF, gWeatherTwoLightsON, gWeatherTwoLightsOff, gLivingroom, gTPLink, gLightPanel,gMorningLightsON, gMorningLightsOFF, gEveningLightsON, gNightLightsOFF) {channel="tplinksmarthome:hs100:8F65C0:switch"}