@dpdoug, You’re not providing much info which allows readers to understand what you have done, or maybe even to replay/reproduce the steps you’ve taken.
Btw, what made you switch from running Grav’s build-in php server, which clearly started your Grav website, to Symfony Local Webserver?
Is it because the “problem page” shown by Grav displayed issues you mentioned in previous posts here and here?
In that case, no matter which webserver you are using, the same “problem page” from Grav will show up. Once you’ve solved the Symfony local webserver issue from this post, you will bump again into your previous issues.
Although I have no experience with Symfony’s webserver (just Googled a bit for this post), I guess the error “# Page nog found” is caused by not starting Symfony from the correct folder.
Btw, when running $ bin/grav server, Grav will use Symfony as first choice if it has been installed properly. Otherwise it will fall back to PHP webserver.