Embedding subtitles ends in ERROR: Conversion failed!

[thrdroom]

Checklist

• I'm reporting a broken site support issue
• I've verified that I'm running youtube-dl version 2019.07.27
• I've checked that all provided URLs are alive and playable in a browser
• I've checked that all URLs and arguments with special characters are properly quoted or escaped
• I've searched the bugtracker for similar bug reports including closed ones
• I've read bugs section in FAQ

Verbose log

./youtube-sync update PursuitofWonder
[youtube:channel] UC-tLyAaPbRZiYrOJxAGB7dQ: Downloading channel page
[youtube:playlist] UU-tLyAaPbRZiYrOJxAGB7dQ: Downloading webpage
[download] Downloading playlist: Uploads from Pursuit of Wonder
[youtube:playlist] UU-tLyAaPbRZiYrOJxAGB7dQ: Downloading page #1
[youtube:playlist] playlist Uploads from Pursuit of Wonder: Downloading 118 videos
[download] Downloading video 1 of 118
[youtube] oGVhOWqsBWM: Downloading webpage
[youtube] oGVhOWqsBWM: Downloading video info webpage
[info] Writing video subtitles to: SYNC/PursuitofWonder/ID/oGVhOWqsBWM.en.vtt
[download] Destination: SYNC/PursuitofWonder/ID/oGVhOWqsBWM.mkv
[download] 100% of 46.87MiB in 00:02
[ffmpeg] Embedding subtitles in 'SYNC/PursuitofWonder/ID/oGVhOWqsBWM.mkv'
ERROR: Conversion failed!
[download] Downloading video 2 of 118
ERROR: Interrupted by user

Description

When i try to download all videos of a channel via youtube-sync i get the error "Conversion failed!" at the step "[ffmpeg] Embedding subtitles in 'SYNC/PursuitofWonder/ID/oGVhOWqsBWM.mkv'". Is this error youtube-dl or ffmpeg related? How to fix this?

The url of the video is https://www.youtube.com/watch?v=oGVhOWqsBWM and the quality setting i use to download is "22/136+bestaudio[ext=m4a]/bestvideo[height<=720]+bestaudio[ext=m4a]/best[height<=720]"

[thrdroom]

@dstftw PLEASE STOP LOCKING ISSUES - i need the ability to ask some questions for clarification PLEASE! What exactly do you mean by "Do not hardcode extension in output template." Do you mean this part "[ext=m4a]" or do you mean the filename-extension ".mkv"? Please help me to find a solution instead of trying to getting rid of me as fast as possible. thanks

[thrdroom]

@dstftw If i shouldn't hardcode the file extension in the output template...how can i force the use of the MKV container instead of mp4 then? How to force MKV without reencoding the video?