Sorry dat ik jullie laat beantwoord,
Ik heb mijn probleem opgelost via onderstaande uitleg,
nogmaals bedankt.
The address you pointed out shows that you are accessing the NAS remotely, via DDNS. As Moody, pointed out, you cannot do that until Plex has been configured.
You must use a local address, EG: http://192.168.0.23:32400 in order to access PMS. If you are still unable to access the server from this local address, then:
Open Plex from your NAS and go to settings->devices. Delete everything except for the current browser session. Stop Plex from running on the NAS.
Next, go to https://plex.tv and sign in if not already. Then click on ‘Launch’ (top right of screen) and repeat the above, delete everything except the current browser session. Then sign out of plex.tv.
Now, go back and restart Plex on your NAS and open it in your browser using http://Your-NAS-IP-Address:32400, sign in, both in settings->server->general and by clicking on the ‘head’ icon (top right of screen).
If the above fails, then follow these steps:
Stop Plex from running, then open File Station and navigate to: Plex/Library/Application Support/Plex Media Server and delete plexmediaserver.pid (if it exists) and Preferences.xml.
If you do not see the Plex folder in File Station, then see Q7a on the Synology FAQ’s page, link in my signature.
Restart Plex, then type http://Your-NAS-IP-Address:32400 into your browser or click on the link in Package Center->Installed->Plex or from the DSM start menu->Plex icon and make sure you log in, both in settings->server->general and by clicking on the ‘head’ icon (top right of screen).
Hopefully, you will be able to access your server again.