I have a Synology DS918+ with DSM 6.2.1-23824 Update 4 and am getting a similar issue. I have tried reinstalling Plex (stop, uninstall, remove preferences.xml, install). I have tried this with the Synology version of Plex and of the latest spk from Plex.tv (PlexMediaServer-1.15.0.659-9311f93fd-x86_64.spk), but I get the same result - “This server is unclaimed and not secure”. I have tried logging out (top right-hand corner) and then hitting “Claim Server” to only end up with this message again. I can change other settings, so I don’t think it is the access level on the preferences file or folder.
To rule out some kind of network/firewall issue, I disabled IPv6, temporarily opened the router’s firewall for the Synology’s IP (inbound/outbound) and disabled all firewall/security options on the NAS, but I still have the same problem.
I am having another issue too where Plex is unable play some MKV files that I can play on a Windows installation of Plex that I was hoping to replace.
Edit: I tried a complete refresh of the Plex installation, deleting the entire Plex folder from the NAS. After reinstalling I am still getting the same issue.
Thanks trumpy81
I went through Q16 and through a number of the others but I don’t think that was the resolution.
I think I have solved it though from reading other posts on this site. I found a clue in a one by ChuckPA where DNS issues were mentioned and also an error that I then found in my own logs.
Feb 21, 2019 16:41:47.511 [0x7f92b0f35700] ERROR - Error issuing curl_easy_perform(handle): 6
Feb 21, 2019 16:41:47.511 [0x7f92b0f35700] WARN - HTTP error requesting GET https://plex.tv/api/v2/features?X-Plex-Token=xxxxxxxxxxxxxxxxxxxxdn’t resolve host ‘plex.tv’)
Feb 21, 2019 16:41:47.511 [0x7f92b0f35700] WARN - FeatureManager: Couldn’t get features. Trying again soon.
Perhaps my DNS resolution was not working properly? I have been using my router address as the DNS. After changing the DS918+ to using the DNS of my ISP instead, it immediately resolved both the issue with the “This server is unclaimed” and also of the playback of the mkv files. When I say immediately, I mean that all I had to do was touch the Plex page…not even a manual refresh. Boom. It seems that my router is no good for DNS resolution, at least not for PLEX anyway.
Thank you for your quick response.
Oh…I think it was it was probably something I did. I was setting it up late one night recently, having just moved house and setting up new network kit. I managed to set some crazy IP and gateway combo that completely locked me out and ended up with me having to factory reset the router to get back in. This issue may have been a legacy from that. I have rebooted everything several times and it did not resolve it.
Now though, I have it all working and Plex on the latest release so I am pretty happy. 
I was thinking of changing the DNS settings anyway as part of setting up for family safety. I have changed the DNS on the router to those recommended by OpenDNS Family Shield and pointed the DS918+ back to the router. For now everything looks to be still working. 
