Brian, this was neither an OpenACS 5.9.* issue, nor a NaviServer issue, but a config file issue. The solution that Maurizio presented is exactly what is present in the recommended default settings shipped with the sample config file for NaviServer (see e.g. [1]).
Since it looks as if your config file was not derived from the sample-config-file, but rather from a legacy configuration, it might be usefule to check for more differences to avoid surprises.
Gustaf, I'm really glad you mentioned that, as I just compared the latest version of the config.tcl with the version I've been using, and they are very different! My config was a modified version of the one included in Maurizio's installer about a month ago, but I hadn't taken Maurizio's latest changes.