For some reason one of my Sanvox sites does not save the password for publishing in the keychain. I moved this to another MAc and it does the same thing. The problem moves with the file. I created the site many versions of Sandovx ago so I dont know if its something corrupted in the file.
Passwords are stored in the keychain which is a system-wide service not under the control of Sandvox. You might need to use the Keychain Assistant application to verify and repair your keychain. Sandvox Help has further hints.