`redirect_uri` is not under the registered domain for this application
I don't know about that Ruby/Gem, but your app registration is not correct. You need to configure your app for explicit OAuth2, per this other answer, except enter localhost instead of example.com. ...
Localhost test Stack App authentication: error 'redirect_uri' does not match
I get the reason: Do not use gem 'omniauth-stackexchange', '~> 0.2.1' ,but instead of gem 'omniauth-stackexchange', git: 'https://github.com/nashby/omniauth-stackexchange.git' The reason see this:...
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