In this case, you’re running the formula “locally” on the server, too, right? Is this in a Web app or something?
You may want to append “?OpenDatabase” to this if it’s a URL. I’m not sure how strict ND6 is here.
It may be a good idea to check whether the resulting filename “looks good”, too. If your users were kept back on R5-structured mail DBs, it’s possible the file name is “.ns5” now.