Okay it’s just that while the symptom is the same, e.g. the proxy isn’t starting, the error is different.
Here it’s…
smart-proxy[977]: /usr/lib/ruby/vendor_ruby/dynflow/director.rb:16:in \`block in <class:Director>': uninitialized constant Concurrent::Edge::Future (NameError)
While in solution you linked to, it’s…
smart-proxy[7075]: /usr/lib/ruby/2.3.0/rubygems/core_ext/kernel_require.rb:55:in `require': cannot load such file -- logging (LoadError)
So while the workaround worked for you, it’s not related to fixing this issue. I think. 
We seem to have overlapping discussion with Service[foreman-proxy]: Systemd restart for foreman-proxy failed!.