The only way I've found so far to get the -1728 error is to have an incorrect userid or password.
127.0.0.1 works (obviously, and for lurkers, if both Indigo Server and the account whose iTunes you are trying to control are on the same computer).
A space in the user name won't work, but you can substitute "%20" for the space.
Interestingly, to test this I changed a long user name (first name only) by adding a space and last initial. That's how I found that space threw an error and the %20 worked. Another script with the old user name also worked. Eight letters in the first name. So maybe only the first eight (or fewer) characters are significant.
Short user name should work, period, so you may have a password error. Try with 127.0.0.1 to eliminate that variable (you get a different error with a bad machine name, though).