Setting TSVar Error in Non-TS environment

Got a weird one here, using UI outside of a task sequence. 

Take this code for an example: 
<UIpp Title="Test" Color="#282c5c" Flat="TRUE">

<Actions>



<Action Type="WMIRead" Variable="PartSN" Namespace="root\cimv2" Class="Win32_systemenclosure" Property="SerialNumber"/>



<Action Type="Info" Title="Serial" Name="SystemName" ShowBack="True" ShowCancel="True">

<![CDATA[<font size="12">Trunked serial = <b>%PartSN%</b></font>]]>

</Action>



</Actions>

</UIpp>


Simples, read WMI, sets variable and displays window with the SerialNumber. 
Works on most of the computers except a few, and I cannot find the reason why. 
It simply doesn't launch, and here's the error: 




I've confirmed WMI is intact on here, my theory is that it can't set the Variable. 
The initial problem was that I couldn't use DefaultValues at all, it wouldn't even produce an error in the log, and just refuse to launch. It would show that it was doing the step but then close the window. 
On a working machine, the step where 'Something went Terribly Wrong' is '+ Set the Task sequence Variable...' 

I have two same model workstations (HP 640 G2), imaged same date with the same TS. Running the same script located on a share. 
One can load UI++, the other gets this 'Something went Terribly Wrong'. 

Does anyone know where it sets the Variables? Maybe a hint to find out how this one machine is different. 
On another workstation (HP 800 G4) that had that the issue, I reimaged and the issue persisted. Would lead to hardware as issue but not model specific. 

Thoughts?