I’m posting this not seeking help, but offering. I have been knocking my head for a couple hours and managed to solve this problem and thought someone else might like to know how.
I am running on RHEL 7 (with system python 2.7.5), with nginx. Certbot installed via yum from system repos.
Recently, everything I tried to do with certbot resulted in the subject error message. Even just
Long story short… looking closely at what I had installed, there were three related packages:
python2-certbot-nginx. I happened to notice that the last of these three had a much lower version number than the other two. I
yum update-d that one, and reinstalled the other two for good luck, and voila, problem fixed.
Hope this helps someone.