Apparently there is firewall software that defaults to filtering HTTP headers it doesn't know about. This breaks redirect-on-post in ajax requests because both Spring-Redirect-URL and Spring-Modal-View would get filtered out.
The problem is that in these cases Spring.Remoting.handleResponse will just fall through without doing anything or giving an error. It finds no redirect URL and the response body is empty so there is nothing to replace.
It would be nice if in this case the error handler would be called or any error at all would be shown. Otherwise finding customer's problems with their broken firewall configuration (or even figuring out that there is a firewall problem) is pretty cumbersome.