dnf now fails every time with the following error: ImportError: /usr/lib/python3.6/site-packages/hawkey/_hawkey.so: undefined symbol: hy_goal_broken_dependency_pkgs
It looks like that your system is broken. I would like to recommend to you to read the article http://dnf.baseurl.org/2016/12/15/repair-of-broken-system-with-dnf-2-0/ and try to recover your system from live cd (reinstall, upgrade, history undo, remove). So far it doesn't look like a problem of dnf but probably if you will provide version of dnf, libdnf ("rpm -q dnf libdnf") just try to reproduce your problem.
I formatted the SD card and reinstalled Fedora 26. The error has not reproduced itself.
Thanks for your report, if you will find any reproduce or again any problem appears, don't hesitate to report it or reopen the bug report. Hope that everything will be fine in future.