This has been annoying me for awhile now. I am running a Lollipop ROM on my OnePlus One, and I have not been able to install any apps by opening their APKs to install directly. It worked fine before (shortly after a fresh install), but now every time I try to open an APK file to install, I get "app not installed" as soon as I click the install button.
I can install and update apps through the Play store just fine. I have plenty of space free for new apps to be installed (more than 10 GB), and I have already allowed installing apps from unknown sources under settings.
I also just found out that Titanium Backup wouldn't restore any of my apps. My current install is still relatively fresh, with some apps restored through Titanium Backup (before it stopped working along with APK installing). I would like to know the cause of this, and I don't want to reinstall just so this will happen again.