I found a solution: I tried to export the APK file and found that it was a problem with my corporate email. When I set up my corporate email (Outlook / Exchange activesync), the phone / tablet receives a security policy from my company that disables "Unknown Sources" in the security settings. "Unknown sources" were grayed out, so I could not turn it on.
To solve this problem, I deleted my corporate email account from the device and policies, and THEN I was able to turn on "Unknown Sources", and then I could finally install APP directly from Eclipse or manually export and install the APK file.
This gives me a new problem. Now I can not read corporate email on my phone :-( If I install the policy again, then I can not install the APK files on my phone: - (
You may have the same problem if you use KNOX, which is standard on new Samsung Galaxy devices.
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