Warum kann ich keine AVI im Media Encoder mehr konvertieren ?

Hard to say why this AVI isn't supported but we do have AVI support on Windows. As you are on Windows our guess is that you are using a codec in an AVI wrapper that we do not support. Please post a screenshot of the output from an app like MediaInfo then we may have a better idea of the exact issue. Please also note though that AVI is definitely not our highest priority right now.