@troelsjohnsen I think the issue was probably with
I was getting the same error until I had mine setup like this...
'facebook' => [ 'client_id' => 'your-facebook-app-id', 'client_secret' => 'your-facebook-app-secret', 'redirect' => 'http://your-registered-call-back-url', ]
I think the confusion is with the 'client_secret' this needs to be your 'App Secret' (yes the one you need to type your password in for).
It's easy enough to assume that the required 'client_secret' for Socialite is the 'Client Token' under the 'Advanced' tab in the Facebook settings, which it isn't.