the same thing happened to me but the solution was a bit simpler than uninstalling imageMagick. It sounds like Rmagick’s config file isn’t updated to use your updated imagemagick so try
sudo gem uninstall rmagick
sudo gem install rmagick
restart your server.
I took a closer look and noticed you had Rmagick configured for a newer imageMagick but using an older imageMagick. So I would assume that my solution would still work but you would not be using the newer ImageMagick.