Error: "The backup file is encrypted, and you have not supplied the correct decryption key for th (4223184)

Error: "The backup file is encrypted, and you have not supplied the correct decryption key for this file." when adding a Subscriber to an existing plan.   Failed to execute operation -  adding a subscriber for an existing, encrypted plan.

Steps to reproduce:
1). Machine (1):  
Create new log shipping (1 publisher and 2 new subscriber -- choose encryption backup file) Setup 5 minutes interval

Example:
Machine (1) : database test1 (publisher)
Machine (2) : database test1 (subscriber) -- read only
Machine (3) : database test1 (subscriber) -- read only

2).  Machine (2): launch LiteSpeed Log Shipping 5.0  and add the third subscriber. The error displays onscreen.