Whose docker image are you using? I highly recommend using something based on Python 3.6. Python 2.7 is literally about to be officially EOL, and there are quite a few outdated libraries which could contribute to this in non-updated, non-recent versions of Python 2.7. I recommend at least Python 2.7.10 if you are using that version. Even then, it's hit or miss.
The official package builds of Curator (RPM/DEB/Windows) are all based on Python 3.6, as it is much more up-to-date, and has many improved SSL library dependencies out of the box.