Thanks for the logs.
I did not see any glaring problems in the log files.
There were no database corruption messages. A corrupt database can interfere with upgrading/downgrading Plex Media Server.
There are two things to look at before trying to update Plex. Neither should interfere with updating Plex, but they are easy to change, so best to to take them off the table.
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Multiple network interfaces on the same subnet can confuse Plex and other applications.
Temporarily disconnect one of the interfaces.
Alternately, you could bond the interface, presenting a unified interface to the network.
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Check for DNS Rebinding Protection in your router. If enabled, disable it.
Also, configure the NAS to use a public DNS server such as 1.1.1.1 (Cloudflare), 9.9.9.9 (Quad9), or 8.8.8.8/8.8.4.4 (Google).
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Uninstall Plex Media Server
After the “are your sure” window, you will be presented with three options. Choose the first, Uninstall Only and keep all settings.
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Now download and install the current public release, 1.32.6.7557 (DSM 7, Intel/AMD 64-bit version).
If DSM refuses to install PMS or the installation does not complete, please note when it failed, take screenshots of any messages, etc. That will help when trying to troubleshoot the failure.
If the installation completes OK, try to access Plex Media Server using the IP address of the server (http://syno_ip_address:32400/web).
If you see the “503 maintenance” message, wait 5 - 10 minutes and try again. Plex is updating the database structure for the new version of PMS.
The web page does not auto-refresh, so close the browser window then try again.
DNS Rebinding
Plex is failing to lookup plex.direct names.
Sep 28, 2023 11:47:15.648 [0x7f3a2f49eb38] ERROR - [EventSourceClient/pubsub/139.162.215.242:443] getaddrinfo(192-168-0-6.abcdefghijklmnopqrstuvwxyz012345.plex.direct) failed: -2
Sep 28, 2023 11:47:15.648 [0x7f3a2f49eb38] DEBUG - [EventSourceClient/pubsub/139.162.215.242:443] Network: 192-168-0-6.abcdefghijklmnopqrstuvwxyz012345.plex.direct failed to resolve to 192.168.0.6 but instead yielded ""
Some routers have DNS rebinding protection enabled. If you can access your router and see such an option, disable it.
The alternative is to use a public DNS server.
See the DNS Rebinding section of How to Use Secure Server Connections for additional info.
Network Interfaces
You have two interfaces active on the same subnet. This can cause problems for Plex and other applications.
It would be best to bond the interfaces together, to present a single interface to the network.
To bond the interfaces, go to Synology Control Panel → Network → Network Interface.
The type of bond will depend on the capability of your switch (if it supports 802.3ad)
If you do not want to bond the interfaces, disconnect one of them, at least temporarily.