I have run into the problem discussed last year where an app finishes downloading but fails to install citing, "Unknown Error Code: -505." The problem was supposedly fixed in Lolipop 5.0.1. My Android device uses the even newer version 5.1. So the error persists.
I should add that the problem only arises after a successful installation that I uninstall and then attempt to reinstall---which I sometimes do for various reasons.
Most recently this incident has occurred with the Guardian Project app "Orbot". However, its sister app, the browser " Orweb", reinstalled just fine.
Finally, sometimes an app will reinstall after multiple attempts. "Orbot" has not been one of them.
How do I get this error to go away?