Hi Francisco,
I understand that you are the SP here.
What the message above means is that the IdP tells the SP that the user
is not allowed to access the server for whatever reason that IdP thinks
is applicable here. The configuration/logfiles of the IdP hopefully show
why that is, so the administrator of the IdP should know why they send
that message.
I would not immediately think of canonicalization as the cause; I take
it that you're using simpleSAMLphp (because you are mailing this list)
and simpleSAMLphp should be handling all the canonicalization and
signatures for you. Unless you have some concrete indications that this
has something to do with it?
Cheers,
Thijs