I suspect that syno-letsencrypt isn’t using the updated version of ACME (from some 6 months ago ) - see Upcoming change: valid authz reuse The client should take note of this and simply obtain a new certificate, however it doesn’t look to be. It’s probably worth raising an issue with syno-letsencrypt
The easiest way may be to change you account key. The domain will not be validated for your new account key, so it should revalidated, and obtain a certificate.