I finally got it running with python3.5 (tried for some time with python 2.7 but gave up).
$ playlistlen /volume1/video/Movies/
Filtering videos: 0%| | 0/4 [00:00<?, ?it/s]
Traceback (most recent call last):
File "/volume1/@appstore/py3k/usr/local/bin/playlistlen", line 11, in <module>
sys.exit(main())
File "/volume1/@appstore/py3k/usr/local/lib/python3.5/site-packages/playlist_length/main.py", line 159, in main
result = video_len_calculator(args.path, args.no_subdir)
File "/volume1/@appstore/py3k/usr/local/lib/python3.5/site-packages/playlist_length/main.py", line 100, in video_len_calculator
path = task.result()
File "/volume1/@appstore/py3k/usr/local/lib/python3.5/concurrent/futures/_base.py", line 398, in result
return self.__get_result()
File "/volume1/@appstore/py3k/usr/local/lib/python3.5/concurrent/futures/_base.py", line 357, in __get_result
raise self._exception
concurrent.futures.process.BrokenProcessPool: A process in the process pool was terminated abruptly while the future was running or pending.