I started receiving the error the full-text query parameter for fulltext query string is not valid after upgrading to .NET 4 when calling a function in SQL Server 2008 from Linq to SQL.   It seems LTS is sending the string as nvarchar(4000) event though I had defined the paramater to the function with a length of nvarchar(200).

The work around that I put in place as we define the parameter to my function as 4000.  Not ideal, but it resolved the error.