Thought I'd put a note on this error. Bearing in mind that this is a general data type error and not just because I tried to convert a date in SQL format to a Month name. 
An error occurred during local report processing.
An error has occurred during report processing.
The Group expression used in grouping 'table1_month' returned a data type that is not valid.
My DataSet Query was as follows: 
SELECT
	CONCAT(MONTHNAME(t1.Date), ' ', YEAR(t1.Date)) Month,
	SUM(IF(t1.VisitorBrowser='MSIE', 1, 0)) 'IE',
	SUM(IF(t1.VisitorBrowser='Firefox', 1, 0)) 'FF',
	SUM(IF(t1.VisitorBrowser='Safari', 1, 0)) 'CS',
	SUM(IF(t1.VisitorBrowser='Smartphone', 1, 0)) 'Smart',
	SUM(IF(t1.VisitorBrowser='Other', 1, 0)) 'Other',
	COUNT(t1.VisitorBrowser) 'Total'
FROM
(SELECT
	a.DateTimeStamp Date,
	CASE
		WHEN INSTR(a.VisitorUAgent, 'MSIE ') THEN 'MSIE'
		WHEN INSTR(a.VisitorUAgent, 'Firefox') THEN 'Firefox'
		WHEN INSTR(a.VisitorUAgent, 'Safari') THEN 'Safari'
		WHEN INSTR(a.VisitorUAgent, 'Chrome') THEN 'Safari'
		WHEN INSTR(a.VisitorUAgent, 'BlackBerry') THEN 'Smartphone'
		WHEN INSTR(a.VisitorUAgent, 'iPhone') THEN 'Smartphone'
		WHEN INSTR(a.VisitorUAgent, 'Android') THEN 'Smartphone'
		WHEN INSTR(a.VisitorUAgent, 'NetFront') THEN 'Smartphone'
		WHEN INSTR(a.VisitorUAgent, 'SonyEricsson') THEN 'Smartphone'
		ELSE 'Other'
	END VisitorBrowser
FROM
	custombu_stats_visits a
GROUP BY
	a.VisitorIP
ORDER BY
	a.DateTimeStamp DESC) t1
GROUP BY
	CONCAT(MONTH(t1.Date), ' ', YEAR(t1.Date))
Caused by SQL statement returning first column date as 
CONCAT(MONTHNAME(t1.Date), ' ', YEAR(t1.Date)) AS Month
Solved by changing SQL statement to 
t1.Date AS Month

This will return the SQL date but using your report builder or business intelligence development studio, you can specify in the report designer how to format the returned value to a more user-friendly date format (such as Month name then year - eg. November 2010).

I have been told that data types are a nightmare in SQL Server Reporting Services and that all data types should be kept in their original state and then queried in the same state.