ID4254: The AttributeValueXsiType of a SAML Attribute must be a string of the form
‘prefix#suffix’, where prefix and suffix are non-empty strings. Parameter name: value
Remove the Issued by claim, resolved the problem.
new Claim("//blah/UserID#",//here the '#' is o.k. "1"//, ConfigurationManager.AppSettings["LocalURLofThisService"] );