Do all the users have the same browser settings in their location document? This is what I’ve seen: If it is an internal site that uses passthru authentication using IIS, a user with settings to use “Notes” as their browser will fail. A user with “Notes with Internet Explorer” as their option will work. It would be tough to see the difference from user perspective – both pages would attempt to open in the Notes client.
(A user with the “Microsoft Internet Explorer” option would also work, of course.)
Hope this is some help.