the upload was in chunks - so that wasn't the problem. I found the solution:
I had to set the parameter 'media_category' to 'tweet_video'.
Please update the documentation on https://dev.twitter.com/rest/reference/post/media/upload-init
currently it only says:
A string enum value which identifies a media usecase. This identifier is used to enforce usecase specific constraints (e.g. filesize, video duration) and enable advanced features.
Please add specific information about this parameter!