I found that I makes a difference whether you install it as a plugin (from github) or a gem. With the gem I could not get the route to accept the :subdomain-condition correctly. But then I installed the plugin with ./script/plugin install…
and all of a sudden it worked fine.
This got me a step closer. When I restarted Apache I got this error message:
undefined method `guard_condition' for class `ActionController::Routing::Optimisation::PositionalArgumentsWithAdditionalParams'
I eventually figured out that this code was added a couple months ago – I removed the code referenced in the blog post and now my routing issues are resolved.