I can't use my backups. "Master Key Issue"

Hello,

Since I forgot my application password, I uninstalled "Remote Desktop Manager" and reinstalled it.
I'm logged in to RDM and I can see my backups in Application.


My goal is to return to the backup that I painted yellow in the image above.
But when I want to return to this backup, I receive the warning message below.


What is master key? I have no information about this. Where can I find the Master Key? Please can you help?

Thanks in advance,