I am using NAV2018. But I have seen this issue in NAV2017 too.
I often back and restore databases to make copies of them to run on the same Server Box, but on different NAV server instance (running on different port nos).
But often I face a problem where the copy of the database refers to the old NAV server instance, and I cannot refresh this. I tried restarting the service and restarting the whole server box. I even checked the event logs, and it says that the service is started successfully.