AppCompat does not support the current theme features - nothing works

To anyone else having this problem, I just found the problem: Apparently there exists another theme with name "AppTheme". I don't know why and how, but changing my theme's name solved the problem.

This was the answer to my problem.

Change:

<item name="android:windowNoTitle">true</item>

to:

<item name="windowNoTitle">true</item>