Why doesn't the Frontend User Login work when clicking "Login" in the Backendmodule?

1. This function only works for backend users with admin rights unless you have enabled the option "Allow non-admin users to log in as frontend users" under Settings → Extension Configuration → Modules in the extension settings.
2. Next, check if the required TypoScript is properly included. The static TypoScript template named "Modules" must be included after your static TypoScript template that defines the content elements.
3. The TypoScript constant that specifies where the frontend users are stored must be defined: themes.configuration.container.frontendUser = 709.
4. Sometimes – for whatever reason – it only works on the second click in the backend.

If everything is set up correctly, your TypoScript should look like this:

frontend_user_login] = PAGE
frontend_user_login = PAGE
    frontend_user_login {
    config {
        debug = 0
        no_cache = 1
        admPanel = 0
    }
    typeNum = 1586608761
    10 = USER
    10 {
        userFunc = TYPO3\CMS\Extbase\Core\Bootstrap->run
        extensionName = Modules
        pluginName = LoginFromBackend
    }
}