I have received the same message... There were no gaps since when you start the online application, it doesn't allow you to proceed onto the next steps until all activity and address history has been provided without gaps.
I sent them a message via WebForm stating that they already were provided all the background info in the application and to provide me a better reason than "replacement needed"; and furthermore told them to base the application based on what was provided earlier.
The message they sent also states: "Please note: The above information/documents must be received within 7 days from the date of this correspondence. If this deadline is not met, your application will be assessed based on the information that was provided with your application and your application could be rejected for being incomplete. FAILURE TO RESPOND TO THIS REQUEST MAY RESULT IN THE REJECTION OF YOUR APPLICATION, NO OTHER REMINDER WILL BE SENT TO YOU "
Why do they come back asking for the same things as if you are just entering the country on a visitor visa?
Anybody on here who did not reply? Or did? And what was the outcome? #IRCCtrolls