TC70 logging crash to file with logcat

We used the following code to log crashes of out app to a logfile:

 

public class App extends Application {

 

   @Override
   public void onCreate() {

 

int stringId = this.getApplicationInfo().labelRes;

String appName = this.getString(stringId);

final String fn = getExternalCacheDir() + "/" + appName + "-log.txt";

File filename = new File(fn);

cmd = "logcat -v time -n 2 -r 1024 -f " + filename.getAbsolutePath();

 

try {

   p = Runtime.getRuntime().exec(cmd);

} catch (IOException e) {
   e.printStackTrace();

}

   }

 

}
The logfile is stored in folder /storage/sdcard0/Android/data/com.tup.crashreport/cache. We developed under Android 4.4.3 on TC50 and everything works as designed.
With TC70 under Android 4.4.2 the logfile is not created (only the folder) and we got the warning "W/ContextImpl: Failed to ensure directory: /storage/sdcard0/Android/data/com.tup.crashreport/cache".
Why does TC70 fail to store the logfile?