Invalid Callback Return Value when returning HTML DIV component

Dash Python
1

Hello,

I am facing a serious issue which has stopped my app from running. I am updating a page after user authentication to switch it from the login page to the content one. the content page is generated by a function. below is the relevant code blocks. For some reason I get a vague error saying:

dash.exceptions.InvalidCallbackReturnValue: The callback for property `children` of component `page-content`
returned a value which is not JSON serializable.

In general, Dash properties can only be dash components, strings,
dictionaries, numbers, None, or lists of those.

despite my return type being <class 'dash_html_components.Div.Div'>

Here is my relevant code blocks:

TAB_VIEW = dcc.Tabs(id='TABS_DISPLAY', children=[TAB0,TAB1,TAB2,TAB3,TAB4])
@validate_login_session
def app_display(VIEW_TYPE):
    return html.Div([
        dcc.Location(id='home-url', pathname='/home'),
        html.Div([
            html.Div([
                html.Button('Tabular View', id='tab-view-button'),
                html.Button('Print View', id='report-view-button'),
            ], style={'display':'inline-block'}),
            html.Div([
                dcc.Link(html.Button('Restart'),  href='/home', style={'display':'inline-block'}),
                dbc.Button('Logout',id='logout-button',color='danger',block=True,size='sm', style={'display':'inline-block'}),
            ], style={'display':'inline-block'}),
        ], style={'display':'flex', 'justify-content':'space-between'}),
        html.Div(id='DISPLAY', children=[VIEW_TYPE])
    ])

################################################################################################
app.layout = html.Div([
        dcc.Location(id='url',refresh=False),
        html.Div(
            id='page-content',
            children=[login_layout()]
        ),
    ])
##################################################################################


##################################################################################
@app.callback(Output('page-content', 'children'),
              [Input('url', 'pathname')])
#              [State('CONTENT1', 'children'),
#              State('CONTENT2', 'children'),
#              State('CONTENT3', 'children')])
def DISPLAY(pathname):
    if pathname == '/home':
        print(type(app_display(TAB_VIEW)))
        return app_display(TAB_VIEW)
#    elif pathname == '/report':
#        PAGE1 = html.Div(id='PAGE1', children=C1)
#        PAGE2 = html.Div(id='PAGE2', children=C2)
#        PAGE3 = html.Div(id='PAGE3', children=C3)
#        REPORT_VIEW =  html.Div([PAGE1,PAGE2,PAGE3])
#        return app_display(REPORT_VIEW)
    elif pathname == '/login':
        return login_layout()
    else:
        return login_layout()

please help, thanks