Errore Data in sql server 2005

ALTER PROCEDURE [dbo].[spLeaveRequestList2]
 @IdUser    int,
 @IdResponsible  int,
 @IdLeaveType  int,
 @LeaFromDate  varchar(14),
 @LeaToDate   varchar(14),
 @IdStatus   int,
 @IdApprovalManager  int,
 @query    varchar(1000)
AS
BEGIN
 -- SET NOCOUNT ON added to prevent extra result sets from
 -- interfering with SELECT statements.
 SET NOCOUNT ON;

 DECLARE @DateStart datetime
 DECLARE @DateEnd   datetime 

 IF @IdUser = -1
  SET @IdUser = 0
 
 IF @IdResponsible = -1
  SET @IdResponsible = 0

 IF @IdLeaveType = -1
  SET @IdLeaveType = 0

 IF @LeaFromDate = ''
  SET @LeaFromDate = '19000202000000'

 IF @LeaToDate = ''
  SET @LeaToDate = '21001231000000'

 IF @IdStatus = -1
  SET @IdStatus = 0

 IF @IdApprovalManager = -1
  SET @IdApprovalManager = 0

 

 SET @DateStart = dbo.StringToData(@LeaFromDate)
 SET @DateEnd   = dbo.StringToData(@LeaToDate)
 
 
IF @QUERY IS NULL

  SELECT  Ler.IDLeaveRequest, 
   Ler.IDUser, 
   Ler.IDLeaveType, 
   dbo.DataToString(Ler.leaFromDate) as leaFromDate, 
   dbo.DataToString(Ler.leaTODate) as leaToDate, 
            Ler.leaAmount, 
   Ler.leaAmountBaseUnit, 
   Ler.leaReason, 
   Ler.leaContactAddress, 
            Ler.leaContactPhone, 
   Ler.leaApproval, 
   isnull(Ler.IDApprovalManager, -1) as IDApprovalManager,
   Ler.IDStatus, 
            Ler.leaManagerNote
  FROM   leaveRequest Ler 
     
    WHERE   Ler.IDUser     =  ISNULL(@IdUser, Ler.IDUser)
   AND   Ler.IDLeaveType       =  ISNULL(@IdLeaveType, Ler.IDLeaveType)
   AND   Ler.leaFromDate    >= @DateStart   
   AND Ler.leaTODate         <= @DateEnd
   AND Ler.IDStatus    =  ISNULL(@IdStatus, Ler.IDStatus)
   AND ((Ler.IDApprovalManager =  ISNULL(@IdApprovalManager, Ler.IDApprovalManager))
   or (@IdApprovalManager is null and Ler.IDApprovalManager is null))
    -- Insert statements for procedure here



 ELSE 


  exec ('SELECT  Ler.IDLeaveRequest, 
   Ler.IDUser, 
   Ler.IDLeaveType, 
   dbo.DataToString(Ler.leaFromDate) as leaFromDate, 
   dbo.DataToString(Ler.leaTODate) as leaToDate, 
            Ler.leaAmount, 
   Ler.leaAmountBaseUnit, 
   Ler.leaReason, 
   Ler.leaContactAddress, 
            Ler.leaContactPhone, 
   Ler.leaApproval, 
   isnull(Ler.IDApprovalManager, -1) as IDApprovalManager,
   Ler.IDStatus, 
            Ler.leaManagerNote
  FROM   leaveRequest Ler 
     
    WHERE   Ler.IDUser     =  ISNULL(' + @IdUser + ', Ler.IDUser)
    AND   Ler.IDLeaveType       =  ISNULL(' + @IdLeaveType + ', Ler.IDLeaveType)
    AND   Ler.leaFromDate    >= ' + @DateStart + '   
    AND Ler.leaTODate         <= ' +  @DateEnd  + ' 
    AND Ler.IDStatus    =  ISNULL(' + @IdStatus + ', Ler.IDStatus)
    AND ((Ler.IDApprovalManager =  ISNULL(' + @IdApprovalManager + ', Ler.IDApprovalManager))
    or (' + @IdApprovalManager + ' is null and Ler.IDApprovalManager is null))
    AND   Ler.IDUser IN (' + @query + ') 
'
)

END

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ALTER FUNCTION [dbo].[StringToData]
(
  @Appo  varchar(20)
)
RETURNS datetime
AS
BEGIN
  
  DECLARE @Data datetime

  SET @Appo = Substring(@Appo,1,4)  + '-' + Substring(@Appo,5,2)  + '-' + 
                Substring(@Appo,7,2) + ' ' + Substring(@Appo,9,2)  + ':' + 
        Substring(@Appo,11,2) + ':' + Substring(@Appo,13,2)  

  SET @Data = convert(datetime, @Appo, 120) 

  -- Return the result of the function
  RETURN @Data

END