Unprocessed XJS sent to client after 301 redirect.

In a new private/incognito browser window, visiting:

http://valhalla.synchro.net/webv4

redirects me (301) to:

http://valhalla.synchro.net/webv4/index.xjs

which sends me said XJS document instead of handling & running it server side. Subsequent visits to the first URL do not have this problem (the browser already knows the new location).

If I open a new incognito window and go straight to http://valhalla.synchro.net/webv4/index.xjs everything works normally.

In the initial case, the browser tells me that the status is 200 but shows no response headers in dev tools; I have not looked any further than that though.