Android Studio suddenly (and for no apparent reason) started complaining that it could not find org.apache.http.legacy (which was added to build.gradle as a legacy library).
Warning: Unable to find optional library: org.apache.http.legacy
The issue was caused by a missing optional.json file that was supposed to be located in Android SDK folder ...AndroidSDK\platforms\android-23\optional
The error was resolved by re-installing Android SDK (v23 - Android 6.0).
Content of the missing optional.json file was:
Android Studio 1.5.1
Android SDK 23