I am using SQL Server Management studio and keep getting the same error, and the only way to get rid of it(usually) is to reset the SQL server(which is very annoying, and so
I had exactly the same problem. It looks like this article was pretty good at solving it. Seems all sorts of buggy things in some versions.
See: You may receive an error message when you try to use SQL Server Management Studio to update a row of a table in SQL Server 2005.
The table contains one or more columns of the text or ntext data type. The value of one of these columns contains the following characters.