[ARD] Mediathek Subtitle format

[a-l-e-k]

• I've verified and I assure that I'm running youtube-dl 2016.04.06
• At least skimmed through README and most notably FAQ and BUGS sections
• Searched the bugtracker for similar issues including closed ones
• Bug report (encountered problems with youtube-dl)

The subtitle in ARD Mediathek is ttml (EBU-TT-D-Basic-DE) but the downloaded file gets the extension srt. In post-processing you get this message:

youtube-dl.exe -v --write-sub --convert-subtitles srt "http://www.ardmediathek.de/tv/FilmMittwoch-im-Ersten/Nur-eine-Handvoll-Leben/Das-Erste/Video?bcastId=10318946&documentId=34251192"
[debug] System config: []
[debug] User config: []
[debug] Command-line args: [u'-v', u'--write-sub', u'--convert-subtitles', u'srt', u'http://www.ardmediathek.de/tv/FilmMittwoch-im-Ersten/Nur-eine-Handvoll-Leben/Das-Erste/Video?bcastId=10318946&documentId=34251192']
[debug] Encodings: locale cp1252, fs mbcs, out cp850, pref cp1252
[debug] youtube-dl version 2016.04.06
[debug] Python version 2.7.10 - Windows-XP-5.1.2600-SP3
[debug] exe versions: ffmpeg N-67100-g6dc99fd, ffprobe N-67100-g6dc99fd, rtmpdump 2.4
[debug] Proxy map: {}
[ARD:mediathek] 34251192: Downloading webpage
[ARD:mediathek] 34251192: Downloading media JSON
[ARD:mediathek] 34251192: Downloading m3u8 information
[info] Writing video subtitles to: Nur eine Handvoll Leben-34251192.de.srt
[debug] Invoking downloader on u'http://hls.daserste.de/i/videoportal/Film/c_610000/610657/format,705238,705219,705237,705220,705208,.mp4.csmil/index_4_av.m3u8'
[download] Nur eine Handvoll Leben-34251192.mp4 has already been downloaded
[download] 100% of 115.69MiB
[ffmpeg] Converting subtitles
[ffmpeg] Subtitle file for srt is already in the requestedformat

When you try to convert the .srt named file to ass or vtt format you get this message:

[ffmpeg] Converting subtitles
ERROR: file: 34251192.de.srt: Invalid data found when processing input