Generation access token for V2 using V1 access key and secret key

You're using the same accessKey/secretKey pair for both your app info and access token. These should actually be different pairs of strings. The app key and secret (a.k.a. consumer key and secret) identify your app. They can be found on the app's page on the App Console. The access token key and secret are different strings that are specific to the user.

So, these two lines should look like this, with the correct and distinct values for each of the four parameter variables:

DbxAppInfo appInfo = new DbxAppInfo(appKey, appSecret);

oAuth1AccessToken = new DbxOAuth1AccessToken(oauth1AccessTokenKey, oauth1AccessTokenSecret);