apt - Can not add repositories in Ubuntu GNOME 17.04

I have tried adding various third party repositories, and I get the same error with all of them.

When I try to add third party repositories through the GUI "Other Software" section, I can open the box to add it, but it does not get added to the list.

When I try to add repositories from Terminal, I get the following output.

pal@pal-Vostro-3546:~$ sudo apt-add-repository -y ppa:teejee2008/ppa
[sudo] password for pal: 
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/softwareproperties/ppa.py", line 358, in get_ppa_info
    ret = get_ppa_info_from_lp(user, ppa)
  File "/usr/lib/python3/dist-packages/softwareproperties/ppa.py", line 94, in get_ppa_info_from_lp
    return get_info_from_lp(lp_url)

  File "/usr/lib/python3/dist-packages/softwareproperties/ppa.py", line 88, in get_info_from_lp
    return _get_https_content_py3(lp_url)
  File "/usr/lib/python3/dist-packages/softwareproperties/ppa.py", line 112, in _get_https_content_py3
    lp_page = urllib.request.urlopen(request, cafile=LAUNCHPAD_PPA_CERT)
  File "/usr/lib/python3.5/urllib/request.py", line 153, in urlopen
    capath=capath)
  File "/usr/lib/python3.5/ssl.py", line 478, in create_default_context
    context.load_verify_locations(cafile, capath, cadata)
ssl.SSLError: unknown error (_ssl.c:3172)


During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/bin/apt-add-repository", line 122, in 
    shortcut = shortcut_handler(line)
  File "/usr/lib/python3/dist-packages/softwareproperties/SoftwareProperties.py", line 864, in shortcut_handler
    ret = factory(shortcut)
  File "/usr/lib/python3/dist-packages/softwareproperties/ppa.py", line 430, in shortcut_handler
    return PPAShortcutHandler(shortcut)
  File "/usr/lib/python3/dist-packages/softwareproperties/ppa.py", line 387, in __init__

    info = get_ppa_info(self.shortcut)
  File "/usr/lib/python3/dist-packages/softwareproperties/ppa.py", line 370, in get_ppa_info
    _get_suggested_ppa_message(user, ppa))
  File "/usr/lib/python3/dist-packages/softwareproperties/ppa.py", line 327, in _get_suggested_ppa_message
    lp_user = get_info_from_lp(LAUNCHPAD_USER_API % user)

  File "/usr/lib/python3/dist-packages/softwareproperties/ppa.py", line 88, in get_info_from_lp
    return _get_https_content_py3(lp_url)
  File "/usr/lib/python3/dist-packages/softwareproperties/ppa.py", line 112, in _get_https_content_py3
    lp_page = urllib.request.urlopen(request, cafile=LAUNCHPAD_PPA_CERT)

  File "/usr/lib/python3.5/urllib/request.py", line 153, in urlopen
    capath=capath)
  File "/usr/lib/python3.5/ssl.py", line 478, in create_default_context
    context.load_verify_locations(cafile, capath, cadata)
ssl.SSLError: unknown error (_ssl.c:3172)

What is the reason for this error, and how can I solve it?