Encrypted folders in Espionage are also "mount points" for files called disk images. These disk images actually contain the encrypted data, and the mountpoint is just a "portal" to access that data.
If you get this error it means that Espionage can't find the folder that's serving as the mount point for the disk image. This means that the folder was either moved or renamed, or was on an external drive that is not currently plugged in.
You can fix this error by plugging in the external drive (if the folder was on an external drive), or click on mount point popup and select a new folder to serve as the mountpoint in the folder details: