Find the solution by myself.
There are two iTunes U sites with different : one is the original iTunes U Public before first activation and another created automatically by the activation process. The last is the actual iTunes U Public site
managed by iTunes U Public Site Manger. The difference is the domain name of the site: if I sign in using my domain name with "-" added at the the end, I can access the actual credentials (Site URL, Debug Suffix, Shared Secret, Administrator Credentials) that differs completely by the ones of the original iTunes site.
No where I found information about that, neither I've received any notification mail about the domain name change after the site activation process.
Hope that can help.