UPDATE: This may now be fixed. Watch this space.
There is an issue with Synology Dynamic DNS calls are returning “Hostname Not Found” errors to their clients.
In reality, most of these calls are actually working (your DNS is being updated). The problem is when a client updates with an IP that is already in place for your host, our side returns No Update Required, which is for some reason being misinterpreted as Hostname Not Found
We’re still working on it, we are also attempting to reach out to somebody on the Synology side to help us work through it.