No soup for you! The server you're trying to access doesn't want to let you in. Make sure you're signed in as a user with access to this server

[tom80H]

Did you run your old server under another user account and migrate the server data directory?
If so… have you already check out this support article?
https://support.plex.tv/articles/204281528-why-am-i-locked-out-of-server-settings-and-how-do-i-get-in/

Alternatively take a look at this post in the tips & tricks section:

Synology FAQ - Questions, Answers, and a few How-To's

FAQ 15: Claiming & Connecting to a troublesome Synology server This FAQ is for those situations where the normal claim process has failed. There are multiple techniques to accomplish this. DSM 7 Uninstall the package with the “Keep” option Reinstall the package , selecting the “Plex Claim Token” option Click “Get Plex Claim Token”. Browser tab will open and display a “claim-xxxxxxx” token string “COPY” the string Return to the installer window PASTE the claim-xxxxx into the small window …