youtube-dl: [ffmpeg] Merging formats into xxxxxx.mp4 ‒ERROR xxxxxxxx.temp.mp4: Invalid argument

Using a fresh OpenSuse Tumbleweed, and fresh youtube-dl, and when trying to

youtube-dl --verbose Lk0GQWlN6kc

I got this error

. . .
[debug] exe versions ffmpeg 2.6.2, ffprobe 2.6.2
. . .
[ffmpeg] Merging formats into Nancy Sinatra - These Boots Are Made for Walkin 1966　[HD 1080p] [Best Quality]-Lk0GQWlN6kc.mp4"
[debug] ffmpeg command line: ffmpeg -y -i Nancy Sinatra - These Boots Are Made for Walkin"' 1966　[HD 1080p] [Best Quality]-Lk0GQWlN6kc.f137.mp4' -i 'Nancy Sinatra - These Boots Are Made for Walkin'" 1966　[HD 1080p] [Best Quality]-Lk0GQWlN6kc.f141.m4a -c copy -map 0:v:0 -map 1:a:0 Nancy Sinatra - These Boots Are Made for Walkin"' 1966　[HD 1080p] [Best Quality]-Lk0GQWlN6kc.temp.mp4'
ERROR: Nancy Sinatra - These Boots Are Made for Walkin' 1966　[HD 1080p] [Best Quality]-Lk0GQWlN6kc.temp.mp4: Invalid argument
Traceback (most recent call last):
  File ./youtube-dl/youtube_dl/YoutubeDL.py, line 1528, in post_process
    files_to_delete, info = pp.run(info)
  File ./youtube-dl/youtube_dl/postprocessor/ffmpeg.py, line 590, in run
    self.run_ffmpeg_multiple_files(info['__files_to_merge'], temp_filename, args)
  File ./youtube-dl/youtube_dl/postprocessor/ffmpeg.py, line 148, in run_ffmpeg_multiple_files
    raise FFmpegPostProcessorError(msg)
FFmpegPostProcessorError

The real output of ffmpeg is:

Unable to find a suitable output format for xxxxxx.mp4

I don’t know whether this is an ffmpeg or youtube-dl issue, perhaps anyone can help. Youtube-dl should in any case show a correct error message, perhaps with a hint, what how to overcome this problem.