Selecting NULL Date Values

[holyone]

I'm not a DBA and I'm sure this is something simple, but I'm just tired of dealing with it. I recently upgraded a client's application DB from MSDE 2000 to SQL Express 2008. A custom report I wrote is throwing errors trying to select fields with NULL date values. The problem is the 'DECISION_DATE' in SELECT. This query ran fine in MSDE 2000. Please excuse my sloppiness:

SELECT 
	g.loan_num AS 'LOAN_NUM', 
	g.borrow_ln AS 'BORR_LAST_NAME',
	ui.last_name AS 'UNDERWRITER',
	u.processor AS 'PROCESSOR',
	g.app_date AS 'DATE_SUBMITTED', 
	u.received AS 'UW_RECEIVED',
	'DECISION_DATE' = 
		CASE 
         WHEN u.suspended IS NULL AND u.approved IS NULL THEN NULL
         WHEN u.suspended IS NULL THEN u.approved
         WHEN u.approved IS NULL THEN u.suspended
         WHEN u.suspended > u.approved THEN u.suspended
         WHEN u.approved > u.suspended THEN u.approved
         ELSE 'ERROR'
      END,
	'STATUS' = 
		CASE 
         WHEN u.suspended IS NULL AND u.approved IS NULL THEN 'NO DECISION'
         WHEN u.suspended IS NULL THEN 'APPROVED'
         WHEN u.approved IS NULL THEN 'SUSPENDED'
         WHEN u.suspended > u.approved THEN 'SUSPENDED'
         WHEN u.approved > u.suspended THEN 'APPROVED'
         ELSE 'ERROR'
      END,
	u.clear_to_close AS 'DATE_CTC',
	DATEDIFF(d,g.app_date,u.received) AS 'DAYS_TO_SUBMIT',
	DATEDIFF(d,u.received,COALESCE(u.suspended,u.approved)) AS 'DAYS_TO_DECISION',
	DATEDIFF(d,g.app_date,u.clear_to_close) AS 'DAYS_TO_CLEAR'
	
FROM 
	DMD_Dataprecision.dbo.doc d, 
	DMD_Dataprecision.dbo.gen g, 
	DMD_Dataprecision.dbo.und u, 
	DMD_Dataprecision.dbo.userinfo ui
WHERE 
	u.file_id = g.file_id AND 
	g.file_id = d.file_id AND 
	u.underwrtr = ui.employ_id AND
		(
			u.processor = 'USER, ONE' OR
			u.processor = 'USER, TWO' OR
			u.processor = 'USER, THREE' OR
			u.processor = 'USER, FOUR'
		) AND
		(
			u.underwrtr <> '008' OR
			u.denied IS NOT NULL OR
				(
					(d.drawn IS NULL) OR 
					(u.approved < d.drawn) OR 
					(u.suspended < d.drawn)
				)
		)
ORDER BY 
	BORR_LAST_NAME ASC

Thank you for your time. Any help or recommendations are much appreciated =)

Oh ya, here's the error:

Msg 241, Level 16, State 1, Line 2
Conversion failed when converting date and/or time from character string.

[rmiao]

What's data type of u.suspended and u.approved? Tried convert them to char in the query?