After looking at the web map JSON for this webmap, I noticed what may be the source of this issue in the renderer;
{
"value": "Customer Owned",
"symbol": {
"color": null,
"size": 0.75,
"angle": 0,
"xoffset": 0,
"yoffset": 0,
"type": "esriSMS",
"style": "esriSMSCircle",
"outline": {
"color": null,
"width": 0.75,
"type": "esriSLS",
"style": "esriSLSNull"
}
},
"label": "Customer Owned"
}
This is the symbol that is displayed for "customer owned" features. I didn't see the error you mentioned when I loaded the map, but I also couldn't render this symbol in the legend widget, probably because the color is null and style of the outline is invalid. I could easily see how this issue could cause the error that you ran into.
I think the best way to fix this is through whichever application was used to set up this renderer in the first place. You can do this in the arcgisonline map viewer.