I use mRemoteNG on a daily basis, but every now and again it crashes, prompting you for a password to load your connections. This is as a result of a corrupt connections files.
Fortunately, the application creates backups of your connections when they change. To fix, navigate to the folder in the error message, in my case this folder is C:\Users\matt\AppData\Roaming\mRemoteNG
In that folder will be a confCons.xml file. This file can be deleted or backed up, and then the last backup can be renamed to confCons.xml. In my case, my last backup file was "confCons.xml.20211006-1534460806.backup" . Restart mRemoteNG, and you should be good to go.