I think your issue is with more than samba, based on the HTTP 404 error. perhaps a traceroute from source to dest may help, since it is a vpn tunnel.
I believe the samba auth box is popping up, because the server cannot be connected to, but it doesn't know if the service is even there to connect to.
as for samba accounts, on the server side, a samba user must be a unix user (or at least I've never seen anyone do otherwise) that has permissions on the linux filesystem that allow them to access the share.
after creating and configuring the unix user, I add them to samba with a line like this:
that will add their unix password to the samba passwords database, and allow them login access to samba shares that are configured with permissions for that user.
sudo smbpasswd -a <username>