E3Kit Error 5 can be read as 'a private key is missing from this device'. This means there's no private key in this device to decrypt data with.
The solution to this problem is to make sure you backed up the private key correctly and that you're restoring it before decrypting data.
Follow the link https://developer.virgilsecurity.com/docs/use-cases/v5/encrypted-communication#whats-next and proceed to the 'Clean Up Device' section.