Oh yes, I've found my mistake.

For the other Noobs:
Developer Mode you must activate in the "Settings->Security" section.

1. After resetting / flashing your device, enable "Allow non-store installations" in the Settings->Programs menu like you did usually.
2. Then, go to Settings->Security menue, activate Developer mode there, confirm the download request, and sit and wait for the restart.

That brings the Terminal back.

Because I haven't needed it for so long time (thanks to the N9QTweak) I simply forgot it.
Best regards