BackendException: SSL failed: [SSL: CERTIFICATE_VERIFY_FAILED]

duplicity 0.7.11
python 2.7.13

When trying to use duplicity with webdavs I get the following error:

BackendException: SSL failed: [SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed (_ssl.c:661)

I tried what is mentioned in "Note on Ssl Certificate Verification" (https://curl.haxx.se/docs/caextract.html) in the doc

--ssl-cacert-file /etc/duplicity/cacert.pem

and

--ssl-cacert-path /etc/duplicity

and

-ssl-no-check-certificate

and I also copied the file to

~/.duplicity/cacert.pem
~/duplicity_cacert.pem
/etc/duplicity/cacert.pem

Nothing worked, still the same message.

I found this thread

https://duplicity-talk.nongnu.narkive.com/behoUdPH/webdav-ssl-certificate-verify-failed

in the end they suggested to use lftp+webdavs instead of webdavs, this
also worked for me. Any ideas why webdavs only does not work?
If something has been deprecated it should be added to the doc.