Repository: yt-dlp
Issue: VrtNu Subtitels not working
Labels: site-bug

### Checklist

- [X] I'm reporting a broken site
- [X] I've verified that I'm running yt-dlp version **2021.11.10.1**. ([update instructions](https://github.com/yt-dlp/yt-dlp#update))
- [X] I've checked that all provided URLs are alive and playable in a browser
- [X] I've checked that all URLs and arguments with special characters are [properly quoted or escaped](https://github.com/ytdl-org/youtube-dl#video-url-contains-an-ampersand-and-im-getting-some-strange-output-1-2839-or-v-is-not-recognized-as-an-internal-or-external-command)
- [X] I've searched the [bugtracker](https://github.com/yt-dlp/yt-dlp/issues?q=) for similar issues including closed ones. DO NOT post duplicates
- [X] I've read the [guidelines for opening an issue](https://github.com/yt-dlp/yt-dlp/blob/master/CONTRIBUTING.md#opening-an-issue)
- [X] I've read about [sharing account credentials](https://github.com/yt-dlp/yt-dlp/blob/master/CONTRIBUTING.md#are-you-willing-to-share-account-details-if-needed) and I'm willing to share it if required

### Region

belgium

### Description

I can't download subtitels from vrtnu, what worked before
i also found that when changing this
https://remix-cf.lwc.vrtcdn.be/remix/eef630ee-06c0-4608-81e4-9f1cbcd309ee/remix_aes.ism/remix_aes-textstream_dut=1000.m3u8
to 
https://remix-cf.lwc.vrtcdn.be/remix/eef630ee-06c0-4608-81e4-9f1cbcd309ee/remix_aes.ism/remix_aes-textstream_dut=1000.vtt

you get subtitels in vtt format

### Verbose log

```shell
[debug] Command-line config: ['-Uv', 'https://www.vrt.be/vrtnu/a-z/de-5e-boog/1/de-5e-boog-s1a1/', '--username', 'PRIVATE', '--password', 'PRIVATE', '--sub-format', 'vtt', '--write-subs']
[debug] Encodings: locale cp1252, fs utf-8, out utf-8, err utf-8, pref cp1252
[debug] yt-dlp version 2021.11.10.1 (win_exe)
[debug] Python version 3.8.10 (CPython 64bit) - Windows-10-10.0.19043-SP0
[debug] exe versions: ffmpeg N-100688-ga423bc9dc2, ffprobe N-100688-ga423bc9dc2
[debug] Optional libraries: Cryptodome, mutagen, sqlite, websockets
[debug] Proxy map: {}
yt-dlp is up to date (2021.11.10.1)
[VrtNU] Logging in
[VrtNU] Requesting XSRF Token
[VrtNU] Performing login
[debug] [VrtNU] Extracting URL: https://www.vrt.be/vrtnu/a-z/de-5e-boog/1/de-5e-boog-s1a1/
[VrtNU] de-5e-boog-s1a1: Downloading webpage
[debug] [Canvas] Extracting URL: https://mediazone.vrt.be/api/v1/vrtvideo/assets/pbs-pub-37e680b6-702d-44fb-aed5-1ad5344d76cc$vid-939f91e1-b5b3-4396-ba9b-c57f74d383ff
[Canvas] pbs-pub-37e680b6-702d-44fb-aed5-1ad5344d76cc$vid-939f91e1-b5b3-4396-ba9b-c57f74d383ff: refreshtoken: Retrieve vrtnutoken
[Canvas] pbs-pub-37e680b6-702d-44fb-aed5-1ad5344d76cc$vid-939f91e1-b5b3-4396-ba9b-c57f74d383ff: Downloading token
[Canvas] pbs-pub-37e680b6-702d-44fb-aed5-1ad5344d76cc$vid-939f91e1-b5b3-4396-ba9b-c57f74d383ff: Downloading video JSON
[Canvas] pbs-pub-37e680b6-702d-44fb-aed5-1ad5344d76cc$vid-939f91e1-b5b3-4396-ba9b-c57f74d383ff: Downloading m3u8 information
[Canvas] pbs-pub-37e680b6-702d-44fb-aed5-1ad5344d76cc$vid-939f91e1-b5b3-4396-ba9b-c57f74d383ff: Downloading m3u8 information
[Canvas] pbs-pub-37e680b6-702d-44fb-aed5-1ad5344d76cc$vid-939f91e1-b5b3-4396-ba9b-c57f74d383ff: Downloading MPD manifest
[debug] Formats sorted by: hasvid, ie_pref, lang, quality, res, fps, hdr:12(7), vcodec:vp9.2(10), acodec, filesize, fs_approx, tbr, vbr, abr, asr, proto, vext, aext, hasaud, source, id
[debug] Downloading subtitles: nl
[debug] Default format spec: bestvideo*+bestaudio/best
[info] pbs-pub-37e680b6-702d-44fb-aed5-1ad5344d76cc$vid-939f91e1-b5b3-4396-ba9b-c57f74d383ff: Downloading 1 format(s): HLS_AES-1282
[info] Writing video subtitles to: De 5e boog - Aflevering 1 (Seizoen 1) [pbs-pub-37e680b6-702d-44fb-aed5-1ad5344d76cc$vid-939f91e1-b5b3-4396-ba9b-c57f74d383ff].nl.vtt
[debug] Invoking downloader on "https://remix-cf.lwc.vrtcdn.be/remix/eef630ee-06c0-4608-81e4-9f1cbcd309ee/remix_aes.ism/remix_aes-textstream_dut=1000.m3u8"
[hlsnative] Downloading m3u8 manifest
[hlsnative] Total fragments: 27
[download] Destination: De 5e boog - Aflevering 1 (Seizoen 1) [pbs-pub-37e680b6-702d-44fb-aed5-1ad5344d76cc$vid-939f91e1-b5b3-4396-ba9b-c57f74d383ff].nl.vtt
[download] De 5e boog - Aflevering 1 (Seizoen 1) [pbs-pub-37e680b6-702d-44fb-aed5-1ad5344d76cc$vid-939f91e1-b5b3-4396-ba9b-c57f74d383ff].nl.vtt.part-Frag3 has already been downloaded
[download]  11.1% of ~16.23KiB at 447.85KiB/s ETA Unknown ETA (frag 3/27)ERROR: Parse error at position 344 (near 'Ik denk dat ik die r')
Traceback (most recent call last):
  File "yt_dlp\YoutubeDL.py", line 1311, in wrapper
  File "yt_dlp\YoutubeDL.py", line 1349, in __extract_info
  File "yt_dlp\YoutubeDL.py", line 1451, in process_ie_result
  File "yt_dlp\YoutubeDL.py", line 1399, in process_ie_result
  File "yt_dlp\YoutubeDL.py", line 2430, in process_video_result
  File "yt_dlp\YoutubeDL.py", line 2627, in process_info
  File "yt_dlp\YoutubeDL.py", line 3552, in _write_subtitles
  File "yt_dlp\YoutubeDL.py", line 2578, in dl
  File "yt_dlp\downloader\common.py", line 399, in download
  File "yt_dlp\downloader\hls.py", line 346, in real_download
  File "yt_dlp\downloader\fragment.py", line 488, in download_and_append_fragments
  File "yt_dlp\downloader\fragment.py", line 464, in append_fragment
  File "yt_dlp\downloader\hls.py", line 261, in pack_fragment
  File "yt_dlp\webvtt.py", line 398, in parse_fragment
yt_dlp.webvtt.ParseError: Parse error at position 344 (near 'Ik denk dat ik die r')
```