I could not upload content the size of a larger post, update the moderation queue, everytime I got a 408 http error from nginx. I reinstalled php, nginx but still nothing worked. Only posts with few text could be published.
Then I waited some hours and finally it's all working. It maybe because of Amazons infrastructure, but I have no idea. So for everyone having suddenly 408 errors uploading content, just wait maybe it goes away.