I was unable to update ddev yesterday due to a Homebrew error. This left ddev unusable. Apparently I had used the macOS system Ruby to install some Ruby gems in the past. Completely uninstalling and reinstalling Homebrew didn’t help. The only fix was to remove the Ruby gems I had installed with the system Ruby:
sudo rm -rf \
/Library/Ruby/Gems/2.6.0/{build_info,cache,doc,extensions,gems} \
/Library/Ruby/Gems/2.6.0/specifications/*.gemspec \
/Library/Ruby/Site
It might have been better to restore the entire Ruby folder from /System/Library/Templates/Data/Library/Ruby. Read this GitHub gist from Claudia Pellegrino for more information.