Not sure why code is failing when CONCAT_NULL_YIELDS_NULL is set to OFF
Try changing your procedure to:

ALTER PROCEDURE dbo.ProcedureName
  @Params ...
AS
BEGIN
  SET NOCOUNT ON;
  SET CONCAT_NULL_YIELDS_NULL ON;

  ... the rest of your code ...
END
GO


You might also want to get in touch with the vendor of the software, as changing this setting to OFF has been deprecated since SQL Server 2005, and soon the setting will have no effect or cause an error:

http://technet.microsoft.com/en-us/library/ms143729(v=sql.90).aspx