Sign in - There was a problem completing this request (Android Device)

Thanks for sharing this. First, please note that you should not use a webview for the Dropbox app authorization flow, per our /oauth2/authorize documentation. 

As for the issue, it looks like it's because it's using an old browser engine, e.g., an old version of Webkit. Unfortunately that is not officially supported by the Dropbox web site. 

Old browsers/engines like this are not officially supported by Dropbox and so may be prone to issues as the Dropbox web site is updated. (This is one of the reasons for our policy prohibiting webviews; we've found webviews are more prone to compatibility issues like this.) For example, it's likely that there's some JavaScript error or incompatibility when attempting to access the Dropbox web site form in this webview, causing the sign in form to fail, as you're seeing. 

Since that isn't supported, I'm afraid we can't offer fixes for issues like this. I recommend you instead upgrade to a supported system browser to resolve this.