Small point, but the links on the duplicacy.com home page are to version 2.1.1. I saw that the Backblaze B2 application key issue was solved in v2.1.2 which is how I knew about the new version.
In the v2.1.1 version, when you put an application id / application key into the GUI fields, the app hangs. This seems to be fixed in v2.1.2.