Cannot switch off File Vault using admin account - "Account cannot be used to manage encryption"

I have been trying to switch off File Vault, however, I get getting "Account “(account name)” cannot be used to manage encryption on this Mac. Click the lock to prevent further changes, then select another administrator account, and try again.

The account I am using is the initial and only admin account. I have tried with another admin account (which I managed to successfully create) however, I am still getting the same error.