T-SQL: Conversion failed when converting date and/or time from character string - tsql

Question

Below is a trigger used to capture updates/inserts on an SQL table. I cannot figure out why, but whenever an update is done, I get the error message Conversion failed when converting date and/or time from character string. Here is the structure of the Transaction Log table:
CREATE TABLE [dbo].[TransactionLog](
[Id] [int] IDENTITY(1,1) NOT NULL,
[TransactionDate] [datetime] NOT NULL,
[Operator] [varchar](35) NOT NULL,
[TableName] [varchar](50) NOT NULL,
[Action] [char](1) NOT NULL,
[TableString] [nvarchar](255) NOT NULL,
[UserId] [char](6) NULL,
CONSTRAINT [PK_TransactionLog] PRIMARY KEY CLUSTERED
(
[Id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON, FILLFACTOR = 90) ON [PRIMARY]
) ON [PRIMARY]
Here is the table being updated:
CREATE TABLE [dbo].[AgentContEd](
[Id] [int] IDENTITY(1,1) NOT NULL,
[sNumber] [int] NOT NULL,
[StateCode] [char](3) NOT NULL,
[CourseCode] [char](6) NOT NULL,
[DateTaken] [date] NOT NULL,
[ExpirationDate] [date] NULL,
[CourseHours] [smallint] NOT NULL,
[Method] [varchar](15) NULL,
[LastChangeOperator] [char](8) NOT NULL,
[LastChangeDate] [datetime] NOT NULL,
[ControlId] [int] NULL,
CONSTRAINT [PK_AgentContEd] PRIMARY KEY CLUSTERED
(
[Id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON, FILLFACTOR = 90) ON [PRIMARY]
) ON [PRIMARY]
And here is the trigger that's causing the headache...
BEGIN
INSERT INTO dbo.TransactionLog
(
TransactionDate,
Operator,
TableName,
Action,
TableString,
UserId
)
SELECT
LastChangeDate,
'Op',
#tableName,
#action,
CAST(
'ID:' + CAST(ISNULL(Id, 'NULL') as char(4))
+ ' SymNum:' + CAST(ISNULL(sNumber, 'NULL') as char(10))
+ ' StateCode:' + ISNULL(StateCode, 'NULL')
+ ' DateTaken:' + CAST(ISNULL(DateTaken, 'NULL') as nvarchar(9))
+ ' ExpDate:' + CAST(ISNULL(ExpirationDate, 'NULL') as nvarchar(9))
+ ' CourseCode:' + ISNULL(CourseCode, 'NULL')
+ ' Hours:' + CAST(ISNULL(CourseHours, 'NULL') as char(3))
+ ' Mthd:' + ISNULL(Method, 'NULL')
As char(255)),
LastChangeOperator
FROM inserted
END

Answer 1

Try
+ ' DateTaken:' + ISNULL(CAST(DateTaken as varchar(9)), 'NULL')
+ ' ExpDate:' + ISNULL(CAST(ExpirationDate as varchar(9)), 'NULL')
I used varchar as it seems pointless to use nvarchar if you are going to be casting the string to char at the end anyway.
Also you probably need to use CONVERT with a style instead of CAST to store something useful. SELECT CAST(getdate() as nvarchar(9)) returns Sep 28 20 for me.
A list of formats is here