Android Studio generated APK installs in debug but not release
I have written a small app in Android Studio which is targeted for API 23 (6.0 Marshmallow) which is what is on my phone. It installs and runs perfectly in debug mode but when I have Android Studio generate a signed APK, the signed APK does not install and has either "App not installed" or "There was a problem parsing the package." I have tried every trick I can find online to no avail. The phone is set to install from unknown sources; the targeted API is what is installed on my phone; and I have not renamed the package. I get the same results from APKInstaller, AndExplorer and just tapping the package in File Manager. Also, I get the same results when the apk is on the SD or internal storage.
I would greatly appreciate help on this problem.