Changing the password has invalidated all access tokens. And that includes the token of your server.
You need to claim the server into your plex user account again.
Here is a script designed to do that GitHub - ChuckPa/UserCredentialReset: User Credential Reset utility for Plex
Here is the item from the Plex Synology FAQ about claiming servers: Synology FAQ - Questions, Answers, and How-To's - #21 by ChuckPa