Simple sample Dash uploader script, sometimes complains 'Failed to fetch'

upload.py

import dash
import dash_core_components as dcc
import dash_html_components as html
from dash.dependencies import Input, Output, State
from flask import Flask

server = Flask(__name__)
app = dash.Dash(server=server)


app.layout = html.Div(
    [
        dcc.Store(id="jobs-store"),
        html.H1("Primer Designer"),
        html.H2("Upload"),
        dcc.Upload(
            id="upload-data",
            children=html.Div(
                ["Drag and drop or click to select a file to upload."]
            ),
            style={
                "width": "100%",
                "height": "60px",
                "lineHeight": "60px",
                "borderWidth": "1px",
                "borderStyle": "dashed",
                "borderRadius": "5px",
                "textAlign": "center",
                "margin": "10px",
            },
            multiple=True,
        ),
        html.Ul(id="file-list"),
    ],
    style={"max-width": "500px"},
)


@app.callback(Output("file-list", "children"), [Input("jobs-store", "data")])
def updatelist(data):
    children = []
    if data is not None:
        children = [html.Li(j) for j in data]
    return children


@app.callback(
    Output("jobs-store", "data"),
    [Input("upload-data", "contents")],
    [State("upload-data", "filename"), State("upload-data", "last_modified"),],
)
def update_output(list_of_contents, list_of_names, list_of_dates):
    """Save uploaded files and regenerate the file list."""

    if list_of_contents is not None:
        children = [
            n
            for c, n, d in zip(list_of_contents, list_of_names, list_of_dates)
        ]
        print(children)
        return children


if __name__ == "__main__":
    app.run_server(debug=True, port=8888, host="172.16.193.97")