This might be the infamous “DNS rebind protection” issue. If you set your server’s DNS server to another one (say, Google’s 8.8.8.8 ), does this still happen?
If you try to connect directly from an external browser to the ip address of the server (so not through app.plex.tv, but for example, http://123.123.123.123:32400/web ), does that work?
(this needs “secure connections” to not be “required” in the PMS Network settings).
maybe reboot the router & server just to start afresh and try the http://[ip address]:[port]/web thing again? (and turn off “enable relay” in PMS Network Settings, so you don’t get relayed/indirect anymore)
To check if remote access is working, from the same network as your server, go to canyouseeme.org. Replace the default port 80 with your remote access port (ex: 32400). It will test if it can reach your Plex server.
To continually monitor remote access, check out uptimerobot.com. It will monitor remote access and send you an e-mail if it cannot reach your server. The free account is all you need. The default monitoring period is every five minutes.
In addition to the suggestions above, it may be worthwhile to see if the remote connection information is being correctly published to Plex’s servers. You can check this by browsing to: https://plex.tv/api/resources?X-Plex-Token=XXXXXX
Replace the Xs with your Plex online token. You can find what your token is using the information here.
This will return an XML document listing all your Plex devices (clients and servers); the server(s) should be listed at the top. You should see a line with your public IP address, with local="0" set. Something like this:
<Connection protocol ="http" address = "123.123.123.123" port = "32400" uri = "http://123.123.123.123:32400" local = "0" />**
Make sure the port shown for this listing matches what you configured for the port forward in your router and what you manually set in the remote access settings in Plex.
Ok, next let’s see if DNS is able to properly resolve your server’s public IP address from its MyPlex FQDN. To figure out what that FQDN is, you need your server’s certificate UUID. You can find it in your registry under: HKEY_CURRENT_USER\SOFTWARE\Plex, Inc.\Plex Media Server. Find the value for CertificateUUID. The FQDN will start with your public IP address (delimited by hyphens instead of periods), the CertificateUUID, and “plex.direct” all separated by periods. So, for example:
123-123-123-123.123457890abcdef.plex.direct
Try pinging that IP address from the remote client to see if it resolves to your public IP address.
Since you tagged Android, I’m assuming you have this issue when using the Android app? Next time you see that message, grab the logs and send those to me. Android, Android TV, Fire TV Logs | Plex Support