One step further I found that the problem is not with while(!Serial), but with the Serial Monitor. I can stable reproduce a behavior of Arduino IDE, where I can force written output to be displayed (after compilation and upload) when I close an opened Serial Monitor window an re-open again or - if not already opened - if I open, close an re-open again. Somehow this window needs to be closed and re-opened for serial connection to be established.
Any thought on that?