By default DBDefence prevent any calls to the procedures. The same is applied when Service Broker calls message handling procedure. You need to exclude handling procedure with security exceptions. Additionally you must consider that before calling your handler SQL Server will try to resolve all referenced object: another stored procedures, functions, tables and queues. Most probably you do not want to list all objects and would like to keep them protected. The solution would be rename original
handler procedure, say, HandleBackup to HandleBackup1, but new HandleBackup will look like that:

create PROCEDURE [dbo].[HandleBackUp]
AS

/* This is not necessary but we check who call the procedure. System
process such as Service Broker do not have login. If we are called by
real user we return because of security reason */

declare @a nvarchar (20)
select @a=original_login()
if (@a <> N'') return

/* End of security check */

// open access to the database
open symmetric key dbdx decryption by password='111'

/* Here we call real procure using late binding. HandleBackup1 will
be resolved and called only after open key statement. */

execute "HandleBackup1"

// close access
close symmetric key dbdx

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