Is there any quick fix? I have upgraded to 1.17.0 recently and now all my Puppet manifests fail because the hashes are malformed? Rails 5.1.7 isn't going to be released anytime soon, so I guess we need a fix for that.
Couldn't we just revert the change that introduced this issue from 1.16.x to 1.17.0?

Well I fixed this in the meantime by manually editing the rails file as mentioned by Michael. Can't we just provide a patched rails release via the foreman repos?

The fix mentioned by Henry Spanka doesnt work for me. (Debian Stretch installed via DEB Repositoy)

@Tobias: Have you restarted foreman (Webserver) after applying the fix? Note that you also need to edit the classes again and just save the parameters again so they are correctly stored in the database. It is not required to change them but you need to resave them.

@Henry: Yes, I've restarted the whole box. After the fix, the array values are displayed fine - without escaped doublequotes - but the puppet agent still failed with the exception above.

I do see it now, with Arrays:

classes:
  dhcp:
    bootfiles:
    - !ruby/hash:ActiveSupport::HashWithIndifferentAccess
      name: foo
      mount_point: "/bar" 
    - !ruby/hash:ActiveSupport::HashWithIndifferentAccess
      name: john
      mount_point: "/doe" 

JSON.parse(@host.info.to_json).to_yaml(:line_width => -1)