Hello,
I recently got my hand on a Synology DS425+,
I was looking to install Plex but as soon the install is done and I create an account and try to link the NAS with the account, I’m got this error message.
Anyone got a lead to how to fix it?
Thanks,
You may want to bookmark Synology FAQ - Questions, Answers, and How-To's
Item 5)
If difficulties continue, use item 15)
In any case: use one of the “big” web browsers without any browser add-ins activated.
Temporarily disable any ad-blockers you may have running in your network.
On edge/Chrome/mozilla/vivaldi/brave all the same

Then Plex is either not running on your NAS, or its firewall is still closed.
ip.addr.of.synoloyg is Not the address! Its the lan IP to your synology. Example: 192.168.0.1 etc
What an idiot i can be sometime.
If done correctly it work