Forms, error on submitting with DdsEncryptedPermanentStorage provider

On enabling the DdsEncryptedPermanentStorage provider, I get the following error on submitting a form:

It is setup with:

I had errors for read access to the keyvault and my key size before, and after fixing those I'm left with the above error.
Am I missing something?

I'm using key 1234567812345678 as base64 MTIzNDU2NzgxMjM0NTY3OA==