Same as @jose.runin , I was having an error due to callback URL mismatch when changing the scheme. However, it seems I had it working by also specifying the scheme in the login call like this:
auth0.webAuthentication(scheme: "demo").login()
(same for logout)
Not sure this is the proper solution though (I am totally new to flutter, so not sure I understand what I am doing here 
)
Thanks for the hint @tyf, much appreciated!