Empty session paths can result in requests being sent to mod_cluster workers that shouldn't be. For example, if a request for apache-served static content has an applicable JSESSIONID (either because the path matches the context or its an empty path and matches everything) with a route valve appended to it, then the static content request is directed to the mod_cluster balancer, resulting in 503s and "All workers are in error state".

So it looks like the presence of a jsessionid.jvmroute cookie causes apache/mod_cluster to improperly route the request even if its a context not handled by any mod_cluster workers.