It's possible that JavaScript is not supported in ArcGIS Server (though I can't find any documentation to confirm that).  Perhaps try changing the parser to Python and rewrite it.  Here's what a more efficient version would look like in Python:
def FindLabel( [NPC_CATEGORY_ID] ):
    npcId = [NPC_CATEGORY_ID]
    idCodes = {
        "231": "WPn53",
        "371": "WMs83",
        "372": "WMs92",
        "375": "WPn53",
        "376": "WPn53"
    }
    if npcId in idCodes:
        return idCodes[npcId]
    return npcId
or, here's what your original function looks like in Python without modification:
def FindLabel( [NPC_CATEGORY_ID] ):
    idArray = ["231","371","372","375","376"]
    codeArray = ["WPn53","WMs83","WMs92","WPn53","WPn53"]
    labelValue = [NPC_CATEGORY_ID]
    for i in range(len(idArray)):
        if idArray == [NPC_CATEGORY_ID]:
            labelValue = codeArray
    return labelValue