You have this check for your re-used conversation handle being either NULL, non-existent or bad state:

IF @dlgId IS NOT NULL

AND NOT EXISTS( SELECT * FROM sys.conversation_endpoints

WHERE conversation_handle = @dlgId AND state IN ('SO', 'CO')

)

But it appears that you only handle the NULL case:

IF @dlgId IS NULL

BEGIN -- Begin the dialog, with the new Id

BEGIN DIALOG CONVERSATION @dlgId

FROM SERVICE [tcp://10.0.0.89:4022/db_ActivityManagement/Audit/DataSender]

TO SERVICE '//Audit/AcMan_DataWriter',

-- this is a MasterAuditDatabase Service Broker Id

-- (change it to yours and remove

'51E3A8D4-BE8B-4162-8B90-245E321F674A'

ON CONTRACT [//Audit/AcMan_Contract]

WITH ENCRYPTION = OFF;

-- add our db's dialog to AuditDialogs table if it doesn't exist yet

INSERT INTO dbo.tbl_AuditDialog(tlg_dbID, tlg_dialogID)

SELECT DB_ID(), @dlgId

END

-- Send our data to be audited

;SEND ON CONVERSATION @dlgId

MESSAGE TYPE [//Audit/AcMan_rMessage] (@AuditedData)

Note that if it is non-existent or in a bad state, nothing is done, but the SEND still runs and tries to use it.