I managed to connect the Saml 2.0 Single Sign On plugin to our enterprise IdP (based on SimpleSAMLphp), but the plugin dies with "A username was not provided.".
In the "Service Provider" tab I configured the mapping for:
Attribute to be used as username uid
Attribute to be used as First Name givenName
Attribute to be used as Last Name sn
Attribute to be used as E-mail mail
and I'm sure that our IdP sends back to all our other SPs these attributes filled with the correct values, but it seems that the plugin is not able to get them, or is not able to apply the mapping I configured.