SQL Server 2012 Error 9512 when modifying user login
Sounds like you have a DDL trigger that is set to listen for ALTER_LOGIN events, and is trying to pass EVENTDATA() directly to some remote procedure.

Check for this using:

USE master;
GO
SELECT OBJECT_DEFINITION([object_id]) 
  FROM sys.server_triggers
  WHERE OBJECT_DEFINITION([object_id]) LIKE N'%ALTER[_]LOGIN%';


It could also be a database-level DDL trigger that listens for ALTER_USER:

USE the_relevant_database;
GO
SELECT OBJECT_DEFINITION([object_id]) 
  FROM sys.triggers
  WHERE parent_class = 0
  AND OBJECT_DEFINITION([object_id]) LIKE N'%ALTER[_]USER%';