Your script works fine for me in OS X 10.2.8. Does this work for you (minor changes made)?
set fspec to (choose file) as text
tell application "URL Access Scripting" to upload file fspec to "ftp://utente:password@server/percorso/completo/" replacing no
I’m just guessing now. Do the selected files have non-ASCII characters in the path? Does this work?
set fspec to (choose file) as Unicode text
tell application "URL Access Scripting" to upload file fspec to "ftp://utente:password@server/percorso/completo/" replacing no