I think this line from SQL Server HELP provides answer. It was a suprise to me. It seems that nvarchar(20) is a different type than nvarchar(26). Interesting at the least....

Try to use dynamic sql for this. For example create such a stored procedure

CREATE PROCEDURE rp_Table

@TableName nvarchar(100)

AS

DECLARE @sql nvarchar(4000)

SET @sql = 'SELECT * FROM ' + @TableName

-- print @sql

EXEC sp_executesql ...

It is not possible to use Reporting Services with MSDE.