Debugging password selv service, ssl error

I am trying to set up the password self service on a standalone UCS 5.0-0 errata127.
When testing with a user on the web */univention/self-service/ I see the following error in the log

Traceback (most recent call last):
  File "/usr/lib/univention-self-service-master/univention-self-service-invitation", line 127, in <module>
    inv = Invitation()
  File "/usr/lib/univention-self-service-master/univention-self-service-invitation", line 84, in __init__
  File "/usr/lib/python2.7/dist-packages/univention/lib/", line 437, in authenticate_with_machine_account
    self.authenticate(username, password)
  File "/usr/lib/python2.7/dist-packages/univention/lib/", line 392, in authenticate
    return self.umc_auth(username, password)
  File "/usr/lib/python2.7/dist-packages/univention/lib/", line 502, in umc_auth
    return self.request('POST', 'auth', data)
  File "/usr/lib/python2.7/dist-packages/univention/lib/", line 532, in request
    return self.send(request)
  File "/usr/lib/python2.7/dist-packages/univention/lib/", line 557, in send
    raise ConnectionError('Could not send request.', reason=exc)
univention.lib.umc.ConnectionError: ('Could not send request.', SSLError(1, u'[SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.c:727)'))

Certificates have already been updated according to Renewing the SSL certificates but the error persists. The UCS shall deliver mail to a mail relay ( with SMTPS (SSL/TLS) )

Any hints how to further debug from this point?

Thanks, Thommie