SAML Attributes require String type

You can rewrite the assertion using an iRule. The rule below might work for you.

when ACCESS_SAML_ASSERTION {
        set assertion [ ACCESS::saml assertion ]       
        set new_assertion [ string map [list "<saml2:AttributeValue xmlns:saml2=\"urn:oasis:names:tc:SAML:2.0:assertion\">" "<saml2:AttributeValue xmlns:saml2=\"urn:oasis:names:tc:SAML:2.0:assertion\" xsi:type=\"xs:string\">"] $assertion ]        
        ACCESS::saml assertion $new_assertion
}