Youtube login returns HTTP Error 400: Bad Request

[floruc]

Checklist

• I'm reporting a broken site support issue
• I've verified that I'm running youtube-dl version 2019.05.20
• 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

florian@mintbox /datastore/data-6/download/trailer $ youtube-dl -v -u xxx@xxx.de -p xxx -o '%(title)s.%(ext)s' :ytwatchlater
[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: [u'-v', u'-u', u'PRIVATE', u'-p', u'PRIVATE', u'-o', u'%(title)s.%(ext)s', u':ytwatchlater']
[debug] Encodings: locale UTF-8, fs UTF-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2019.05.20
[debug] Python version 2.7.12 (CPython) - Linux-4.15.0-50-generic-x86_64-with-LinuxMint-18.3-sylvia
[debug] exe versions: ffmpeg 2.8.15, ffprobe 2.8.15
[debug] Proxy map: {}
[youtube:watchlater] Downloading login page
[youtube:watchlater] Looking up account info
[youtube:watchlater] Logging in
WARNING: Unable to log in: HTTP Error 400: Bad Request
[youtube:watchlater] WL: Downloading webpage
ERROR: This playlist does not exist.
Traceback (most recent call last):
  File "/usr/local/bin/youtube-dl/youtube_dl/YoutubeDL.py", line 796, in extract_info
    ie_result = ie.extract(url)
  File "/usr/local/bin/youtube-dl/youtube_dl/extractor/common.py", line 529, in extract
    ie_result = self._real_extract(url)
  File "/usr/local/bin/youtube-dl/youtube_dl/extractor/youtube.py", line 3077, in _real_extract
    _, playlist = self._extract_playlist('WL')
  File "/usr/local/bin/youtube-dl/youtube_dl/extractor/youtube.py", line 2589, in _extract_playlist
    raise ExtractorError(message, expected=True)
ExtractorError: This playlist does not exist.

Description

I regulary use youtube-dl to download videos saved in my Youtube Watch later playlist. For some days the login process of youtube-dl seems to fail and returns "HTTP error 400: Bad request". I tried logging out and in again via Firefox. This works fine but doesn't help for youtube-dl.