InertiaJS, Laravel, Double request fired with 409 HTTP Status, after to_route

Hey,

I run into this strange issue after updating a ressource from my frontend via any Inertia.put/post/delete request.

output in the browser's dev console:

Uncaught (in promise) TypeError: NetworkError when attempting to fetch resource.

The telescope output:

GET 	/users 	200 	181ms 	6m ago 	
GET 	/users 	409 	163ms 	6m ago 	<-- suspicious second request
PUT 	/users/update 	303 	192ms 	6m ago 	

suspicious second request:
Response:
"Empty Response"

Headers:
"host": "127.0.0.1:8000",
"accept": "text/html, application/xhtml+xml",
"accept-language": "de,en-US;q=0.7,en;q=0.3",
"accept-encoding": "gzip, deflate, br, zstd",
"x-requested-with": "XMLHttpRequest",
"x-inertia": "true",
"x-xsrf-token": "********",
"referer": "http://127.0.0.1:8000/user",
"connection": "keep-alive",
"cookie": "********",
"sec-fetch-dest": "empty",
"sec-fetch-mode": "cors",
"sec-fetch-site": "same-origin",
"priority": "u=0",
"pragma": "no-cache",
"cache-control": "no-cache"

Component.vue

Inertia.put(route('user.update'), form, {
            onSuccess: () => {
                closeDialog()
            },
        })

web.php

Route::get('/', [UsersController::class, 'index'])->name('user.index');
Route::put('/update', [UsersController::class, 'update'])->name('user.update');

UsersController.php

public function update(Request $request)
    {
        $user = User::find($request->id);

        $sUserEmail = $request->Email;
        $sUserPassword = $request->password;

        $user->update ([
           ....
        ]);

        return to_route('user.index'); <--- the redirect from the official inertia docs
    }

public function index(): Response
    {

        $users = User::with('UserType')->get();

        return Inertia::render('user/Index', [
            'users' => $users ,
            
        ]);
    }

How should I deal with it? Why does it happen? I read about Inertia forcing the frontend to update if versions don't match however that should happens once as far as I know.

Any ideas?