Thanks for the log. I see that you're using a relatively old version of the product which means you are using the low-level API.
The code below is from the AssertionExample project that we include and that you should have. It's in VB.NET but if you're using C# hopefully it's easy enough to follow the syntax.
The important thing to note is call to SAMLAttribute.RegisterAttributeValueSerializer. You will need to make a similar call so the SAML API knows to treat the attribute value as XML rather than as a string. The high-level API handles this automatically for you but with the low-level API you need to do this yourself.
Please include this call in your code. If there's still an issue, please send me a section of code demonstrating the issue. Thanks.
' Include an attribute containing XML as its value.
' To correctly serialize/deserialize the value as XML rather than a string,
' the correct serializer must be registered for the attribute.
' In this example the Address attribute is XML.
SAMLAttribute.RegisterAttributeValueSerializer("Address", Nothing, New XmlAttributeValueSerializer())
Dim addressAttribute As New SAMLAttribute("Address", SAMLIdentifiers.AttributeNameFormats.Unspecified, Nothing)
Dim xmlDocument As New XmlDocument()
xmlDocument.PreserveWhitespace = True
xmlDocument.LoadXml("<Address><Street>1234 Main Street</Street><Town>Any Town</Town><Zip>56789</Zip></Address>")
attributeStatement = New AttributeStatement()