In tappAS 0.99.15, we update some sources and how tappAS works with the annotations files. If you don't have the lasted version of tappAS your next projects downloading a new annotation will failed. By other hand, your old projects wouldn't worked with new annotations by the same change.
If you want to update tappAS, you have to delete your old projects and delete your References folder in your tappasWorkspace before run the new version. Then, you can create your old projects again and they will work with the new changes.
Remember that tappAS is still a beta version and changes like this could happen in the future. If you want to update tappAS to version 0.99.15 or lasted and you have a lower version can follow this link.