The blog portion of my Middleman website builds successfully if I'm using a ruby version of 2.2.x (specifically I've used 2.2.1 and 2.2.4). The build fails with the following error if you're on 2.3.1 or 2.4.0:
/Users/myname/.rvm/gems/ruby-2.3.1/gems/middleman-core-4.1.2/lib/middleman-core/sitemap/extensions/proxies.rb:89:intarget_resource': Path blog/2017/04/11.html proxies to unknown file blog/calendar.html`
The tag.html.erb and calendar.html.erb files are definitely in the location referenced by this error (and it has very sporadically built on occasion if I toggle a bunch of settings in my config.rb blog settings).
The README for middleman-blog states that it supports 2.2.4, 2.3.1, and 2.4.0, but I also see a commit in March of 2017 that updates the travis.yml to include the 2.3 and 2.4 versions... which makes me wonder how much those versions have been tested/used to this point.
My ruby/rails-fu is at best novice, but I suspect that I can just pin this ruby version (2.2.4 or ~>2.2.0 or something like that) to the Gemfile and work around this in the meantime as it builds very reliably on versions < 2.3.0.