You want to set up a new Plex server on the Synology NAS, but after logging in you only get the message “Access denied” instead of the server setup.
You are probably not accessing the server from the same IP address range.
For example, during setup I accessed a server from a client with IP 192.168.4.7, while the server could be reached on IP 192.168.5.6.
Although routing between address ranges seems to be correct – because you can access the website itself – a security feature of the Plex server ensures that no connection can be established. This is to prevent that fresh Plex installations are hijacked from foreign network areas.
So make sure that you are accessing from the same address range. In my case, everything worked right away with a “neighboring” IP 192.168.5.20. Unfortunately, at this point I had already been searching for the error for 3 hours in vain.