Sorry to jump in but I found this topic is interesting.
I tried follow the step by this post Windows-Legacy - Help / Studio - UiPath Community Forum. Then download UiPath.FTP.Activities[v2.3.0] and UiPath.Credentials.Activities[v2.0.0], which should be ready at your project.
Then I could open the xaml without error.
Just saw the J0ska’s update. Should be the same.