Architecture: 64-bit x86.

With --verbose:

penguin% youtube-dl --verbose "https://www.youtube.com/watch?v=CU3mc0yvRNk"
[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: ['--verbose', 'https://www.youtube.com/watch?v=CU3mc0yvRNk']
[debug] Encodings: locale UTF-8, fs utf-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2018.09.01
[debug] Python version 3.7.0 (CPython) - Linux-4.18.5-gnu-1-x86_64-with-arch
[debug] exe versions: ffmpeg n4.0.2, ffprobe n4.0.2, rtmpdump 2.4
[debug] Proxy map: {}
[youtube] CU3mc0yvRNk: Downloading webpage
[youtube] CU3mc0yvRNk: Downloading video info webpage
WARNING: unable to extract html5 player; please report this issue on https://yt-dl.org/bug . Make sure you are using the latest version; see https://yt-dl.org/update on how to update. Be sure to call youtube-dl with the --verbose flag and include its complete output.
[youtube] {43} signature length 41.42, html5 player None
[youtube] CU3mc0yvRNk: Downloading player https://www.youtube.com/yts/jsbin/player_remote_ux-vfl8oKw17/en_US/base.js
ERROR: Signature extraction failed: Traceback (most recent call last):
File "/usr/lib/python3.7/site-packages/youtube_dl/extractor/youtube.py", line 1210, in _decrypt_signature
video_id, player_url, s
File "/usr/lib/python3.7/site-packages/youtube_dl/extractor/youtube.py", line 1121, in _extract_signature_function
res = self._parse_sig_js(code)
File "/usr/lib/python3.7/site-packages/youtube_dl/extractor/youtube.py", line 1182, in _parse_sig_js
jscode, 'Initial JS player signature function name', group='sig')
File "/usr/lib/python3.7/site-packages/youtube_dl/extractor/common.py", line 972, in _search_regex
raise RegexNotFoundError('Unable to extract %s' % _name)
youtube_dl.utils.RegexNotFoundError: Unable to extract Initial JS player signature function name; please report this issue on https://yt-dl.org/bug . Make sure you are using the latest version; see https://yt-dl.org/update on how to update. Be sure to call youtube-dl with the --verbose flag and include its complete output.
(caused by RegexNotFoundError('Unable to extract \x1b[0;34mInitial JS player signature function name\x1b[0m; please report this issue on https://yt-dl.org/bug . Make sure you are using the latest version; see https://yt-dl.org/update on how to update. Be sure to call youtube-dl with the --verbose flag and include its complete output.')); please report this issue on https://yt-dl.org/bug . Make sure you are using the latest version; see https://yt-dl.org/update on how to update. Be sure to call youtube-dl with the --verbose flag and include its complete output.
Traceback (most recent call last):
File "/usr/lib/python3.7/site-packages/youtube_dl/extractor/youtube.py", line 1210, in _decrypt_signature
video_id, player_url, s
File "/usr/lib/python3.7/site-packages/youtube_dl/extractor/youtube.py", line 1121, in _extract_signature_function
res = self._parse_sig_js(code)
File "/usr/lib/python3.7/site-packages/youtube_dl/extractor/youtube.py", line 1182, in _parse_sig_js
jscode, 'Initial JS player signature function name', group='sig')
File "/usr/lib/python3.7/site-packages/youtube_dl/extractor/common.py", line 972, in _search_regex
raise RegexNotFoundError('Unable to extract %s' % _name)
youtube_dl.utils.RegexNotFoundError: Unable to extract Initial JS player signature function name; please report this issue on https://yt-dl.org/bug . Make sure you are using the latest version; see https://yt-dl.org/update on how to update. Be sure to call youtube-dl with the --verbose flag and include its complete output.
Traceback (most recent call last):
File "/usr/lib/python3.7/site-packages/youtube_dl/extractor/youtube.py", line 1210, in _decrypt_signature
video_id, player_url, s
File "/usr/lib/python3.7/site-packages/youtube_dl/extractor/youtube.py", line 1121, in _extract_signature_function
res = self._parse_sig_js(code)
File "/usr/lib/python3.7/site-packages/youtube_dl/extractor/youtube.py", line 1182, in _parse_sig_js
jscode, 'Initial JS player signature function name', group='sig')
File "/usr/lib/python3.7/site-packages/youtube_dl/extractor/common.py", line 972, in _search_regex
raise RegexNotFoundError('Unable to extract %s' % _name)
youtube_dl.utils.RegexNotFoundError: Unable to extract Initial JS player signature function name; please report this issue on https://yt-dl.org/bug . Make sure you are using the latest version; see https://yt-dl.org/update on how to update. Be sure to call youtube-dl with the --verbose flag and include its complete output.

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/usr/lib/python3.7/site-packages/youtube_dl/YoutubeDL.py", line 792, in extract_info
ie_result = ie.extract(url)
File "/usr/lib/python3.7/site-packages/youtube_dl/extractor/common.py", line 502, in extract
ie_result = self._real_extract(url)
File "/usr/lib/python3.7/site-packages/youtube_dl/extractor/youtube.py", line 1803, in _real_extract
encrypted_sig, video_id, player_url, age_gate)
File "/usr/lib/python3.7/site-packages/youtube_dl/extractor/youtube.py", line 1220, in _decrypt_signature
'Signature extraction failed: ' + tb, cause=e)
youtube_dl.utils.ExtractorError: Signature extraction failed: Traceback (most recent call last):
File "/usr/lib/python3.7/site-packages/youtube_dl/extractor/youtube.py", line 1210, in _decrypt_signature
video_id, player_url, s
File "/usr/lib/python3.7/site-packages/youtube_dl/extractor/youtube.py", line 1121, in _extract_signature_function
res = self._parse_sig_js(code)
File "/usr/lib/python3.7/site-packages/youtube_dl/extractor/youtube.py", line 1182, in _parse_sig_js
jscode, 'Initial JS player signature function name', group='sig')
File "/usr/lib/python3.7/site-packages/youtube_dl/extractor/common.py", line 972, in _search_regex
raise RegexNotFoundError('Unable to extract %s' % _name)
youtube_dl.utils.RegexNotFoundError: Unable to extract Initial JS player signature function name; please report this issue on https://yt-dl.org/bug . Make sure you are using the latest version; see https://yt-dl.org/update on how to update. Be sure to call youtube-dl with the --verbose flag and include its complete output.
(caused by RegexNotFoundError('Unable to extract \x1b[0;34mInitial JS player signature function name\x1b[0m; please report this issue on https://yt-dl.org/bug . Make sure you are using the latest version; see https://yt-dl.org/update on how to update. Be sure to call youtube-dl with the --verbose flag and include its complete output.')); please report this issue on https://yt-dl.org/bug . Make sure you are using the latest version; see https://yt-dl.org/update on how to update. Be sure to call youtube-dl with the --verbose flag and include its complete output.

I fixed this on my end by updating youtube-dl to the latest. The one in Parabola's repos was behind, so I had to use pip to get the latest:

pip3 install --upgrade youtube-dl