Reason for this error message:
1) Comparing XML Data type Columns/Variables.
2) Sorting based on XML Data types

Example Queries:
[sql]
DECLARE @A XML
DECLARE @B XML

SELECT 1 WHERE @A = @B
[/sql]

Fix/Resolution:
1) Check if the query contains direct comparisions based on XML Data type columns/Variables.
1) Check if the query uses xml datatype in the order by clause.
3) Cast the XML Datatypes to varchar(or some other compatable form) and Compare/sort.
2) Use xquery to Compare/sort XML Data types values indirectly.

This error comes up when you try to set XML variable with an int value.

Sample Code:
[sql]
DECLARE @Var1 XML
DECLARE @Var2 INT

SET @Var2 = 1

SELECT @Var1 = @Var2
[/sql]

When you execute the above code you get this error at line 6
SELECT @Var1 = @Var2

Fix:
Check all the XML related variables, columns in the table, Arguments to stored procedure, function etc…
and validate if the correct XML value is set.

Applicable to SQL Server 2000,SQL Server 2005,SQL Server 2008,SQL Server 2008 r2,SQL Server 2012, SSIS