File.OpenBinaryDirect throws error 401/403 in SharePoint 2013 Provider Hosted App (Add-Ins)

I was trying to get a file from a SharePoint document library (host-web) with the .NET Client Object Model (CSOM) in a provider hosted app. I used the OpenBinaryDirect method from the File-class. Code Snippet List spList = ctx.Web.Lists.GetByTitle(“Dokumente”); CamlQuery camlQuery = new CamlQuery {ViewXml = String.Format(@”<View>          <Query>             <Where><Eq><FieldRef Name=’FileLeafRef’ /><Value Type=’File’>{0}</Value></Eq></Where>        …