I've checked and the app is still listed on the Google Play Store. The 403 error actually means Forbidden and it's directly related to the right to access the resource. This can happen if the application has been blacklisted by Instagram developers, or you are blocked by the system for attempting to download a large number of pictures in a very short time. You should just way awhile, because in most of the cases, it goes away and you can start over.
There is also the possibility that the app you're using to require an update to work with the new security standards implemented by Instagram developers.