Ruby API v2 Client got Internal Server Error after redirected from oauth2.authorize

[Togor]

I deployed my Ruby app in GAE, and browsed it with url: https://xxxxxxxxxxxxxx.appspot.com/.

After directed to https://www.dropbox.com/oauth2/authorize, I click the Allow, the the page was directed to 

https://xxxxxxxxxxxxxx.appspot.com/auth, and only showed: Internal Server Error

Part of my code is pasted below:

def authenticator
    # Create dropbox session object and serialize it to the Sinatra session
    authenticator = DropboxApi::Authenticator.new(APP_KEY, APP_SECRET)
end

enable :sessions

get '/' do
    @loggedin = (session[:dropbox] != nil)
    erb :index
end

get '/login' do
    session[:dropbox] = 'init'
    # redirect user to Dropbox auth page
    redirect authenticator.authorize_url :redirect_uri => URL #= dropbox_session.get_authorize_url(URL)
end

get '/auth' do

    redirect 'login' unless session[:dropbox] == 'init'

    auth = authenticator.get_token(params['code'], :redirect_uri => URL)
    token = auth.token

    session[:dropbox] = token
    redirect '/write'
end

This is the log: 

2020-07-05 11:47:50 default[20200705t193819] https://xxxxxxxxxxxxxx.appspot.com/ -> /login
2020-07-05 11:48:47 default[20200705t193819] "GET /write?code=jezOSqKvFNAAAAAAAAAAT42cxSNkV3SGaY2HhBnt-28 HTTP/1.1" 302
2020-07-05 11:48:47 default[20200705t193819] 112.120.226.172, 169.254.1.1 - - [05/Jul/2020 11:48:47] "GET /write?code=jezOSqKvFNAAAAAAAAAAT42cxSNkV3SGaY2HhBnt-28 HTTP/1.1" 302 - 0.0007
2020-07-05 11:48:47 default[20200705t193819] 127.0.0.1 - - [05/Jul/2020:11:48:47 UTC] "GET /write?code=jezOSqKvFNAAAAAAAAAAT42cxSNkV3SGaY2HhBnt-28 HTTP/1.1" 302 0
2020-07-05 11:48:47 default[20200705t193819] https://xxxxxxxxxxxxxx.appspot.com/ -> /write?code=jezOSqKvFNAAAAAAAAAAT42cxSNkV3SGaY2HhBnt-28
2020-07-05 11:48:48 default[20200705t193819] "GET /auth HTTP/1.1" 500
2020-07-05 11:48:49 default[20200705t193819] OAuth2::Error - invalid_request: "code": must not be empty
2020-07-05 11:48:49 default[20200705t193819] {"error_description": "\"code\": must not be empty", "error": "invalid_request"}:
2020-07-05 11:48:49 default[20200705t193819] /layers/google.ruby.bundle/gems/.bundle/gems/ruby/2.5.0/gems/oauth2-1.3.1/lib/oauth2/client.rb:119:in `request'
2020-07-05 11:48:49 default[20200705t193819] /layers/google.ruby.bundle/gems/.bundle/gems/ruby/2.5.0/gems/oauth2-1.3.1/lib/oauth2/client.rb:146:in `get_token'
2020-07-05 11:48:49 default[20200705t193819] /layers/google.ruby.bundle/gems/.bundle/gems/ruby/2.5.0/gems/oauth2-1.3.1/lib/oauth2/strategy/auth_code.rb:30:in `get_token'
2020-07-05 11:48:49 default[20200705t193819] app.rb:44:in `block in <main>'

[Greg-DB]

The '"code": must not be empty' comes from the Dropbox API /oauth2/token endpoint, and indicates that the client did not supply the required "code" parameter. 

You'll need to debug your app to see why you're not sending up the expected code value, which seems like it's supposed to be in the 'params['code']' in your case. I can't offer help with Ruby or your web framework itself though.