After updating my nexus 4 to new android 5 (lollipop) when i open an apk file, the install button not working!! Also i checked unknown source in my device settings.
What the problem? please help :(
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This happens when you have apps that interfere with the installation.
Example: I used to have Avast on my phone. Every now and then I couldn't install an app for the problem you're describing.
I uninstalled Avast (but just disabling it would be fine) and everything went fine.
So this is it: look at the apps already installed in your phone. Chances are there is something that blocks the installation...