I believe the portion after the dot is a signature that is computed from the account key by the ACME client. It's possible there's some bug in the Vesta client that is mixing up account keys or otherwise not creating this signature properly.
Other people have reported this bug as well:
Since there seems to be no resolution to the issue, I would suggest taking a backup of your current certificate and private key and then doing whatever you can to erase or reset the Let's Encrypt configuration so that it can register a new account key and start fresh.