Thank you very much for your work!

Okay, thank you very much for this information.

Okay I have uploaded a new version of the video and thumbnail is created now. Thanks for your help!

Fine! As far as I know this parameter is used to set the maximum interval between keyframes. So the real size of intervals will be different from this value. Why do you prefer a value of 128? I think it should depend on the framerate of the input video. Otherwise it will produce in a 60fps file a new keyframe after about 2 seconds and in a 24fps file after about 5 seconds (theoretical maximum).

Thanks for your examination of the problem. Yes, the seeking issue here is also reproducibly . I wish you success in solving the problem!

Well it is transcoded from a lossless avi by ffmpeg (two pass transcoding) with ""ffmpeg.exe" -i infile -c:v libvpx-vp9 -pass 1 -aspect 16:9 -b:v 12000K -crf 10 -threads 8 -speed 4 -tile-columns 6 -frame-parallel 1 -an -f webm
ffmpeg.exe" -i infile -c:v libvpx-vp9 -pass 2 -b:v 12000K -crf 10 -aspect 16:9 -threads 8 -speed 2 -tile-columns 6 -frame-parallel 1 -auto-alt-ref 1 -lag-in-frames 25 -an -f webm outfile.webm"
I also used these settings on a few other but shorter (e.g. File:Trichterspinne mit Beute -2015-06-14.webm) uploads. In fact the creation of the thumbnail seems to take a significant longer time than using the vp8 codec.

Sorry, being a normal commons and wikipedia user I am not so familiar with phabricator. Well, please have a look to https://commons.wikimedia.org/wiki/File:VID_20150327_164408.webm. In the "Transcode status" section of the file you will find something like "Added to Job queue 1 day, 8 hours, 39 minutes, 3 seconds ago". Trying ''Reset transcode'' doesn't solve the problems. And all videos uploaded later on show this behaviour. .