I enabled the syslog settings in your posted link. The plex library has disappeared again. I don't see anything in the syslog about plex, but it does say that another docker, NZBGet, has crashed for some reason.
Aug 5 00:25:30 Vault kernel: docker0: port 4(veth954b6bd) entered forwarding state
Aug 5 00:29:33 Vault kernel: docker0: port 4(veth954b6bd) entered disabled state
Aug 5 00:29:33 Vault kernel: vethce454a9: renamed from eth0
Aug 5 00:29:33 Vault kernel: docker0: port 4(veth954b6bd) entered disabled state
Aug 5 00:29:33 Vault kernel: device veth954b6bd left promiscuous mode
Aug 5 00:29:33 Vault kernel: docker0: port 4(veth954b6bd) entered disabled state
Aug 5 00:33:21 Vault kernel: igc 0000:02:00.0 eth0: NIC Link is Down
Aug 5 00:33:21 Vault kernel: bond0: (slave eth0): link status definitely down, disabling slave
Aug 5 00:33:21 Vault kernel: device eth0 left promiscuous mode
Aug 5 00:33:21 Vault kernel: bond0: now running without any active interface!
Aug 5 00:33:21 Vault kernel: br0: port 1(bond0) entered disabled state
Aug 5 00:33:23 Vault dhcpcd[1121]: br0: carrier lost
Aug 5 00:33:23 Vault avahi-daemon[5897]: Withdrawing address record for 192.168.1.232 on br0.
Aug 5 00:33:23 Vault avahi-daemon[5897]: Leaving mDNS multicast group on interface br0.IPv4 with address 192.168.1.232.
Aug 5 00:33:23 Vault avahi-daemon[5897]: Interface br0.IPv4 no longer relevant for mDNS.
Aug 5 00:33:23 Vault dhcpcd[1121]: br0: deleting route to 192.168.1.0/24
Aug 5 00:33:23 Vault dhcpcd[1121]: br0: deleting default route via 192.168.1.254
Aug 5 00:33:23 Vault dnsmasq[7969]: no servers found in /etc/resolv.conf, will retry
Aug 5 00:33:25 Vault ntpd[1294]: Deleting interface #1 br0, 192.168.1.232#123, interface stats: received=70, sent=70, dropped=0, active_time=25005 secs
Aug 5 00:33:25 Vault ntpd[1294]: 216.239.35.8 local addr 192.168.1.232 -> <null>
Aug 5 00:33:57 Vault kernel: igc 0000:02:00.0 eth0: NIC Link is Up 1000 Mbps Full Duplex, Flow Control: RX/TX
Aug 5 00:33:57 Vault kernel: bond0: (slave eth0): link status definitely up, 1000 Mbps full duplex
Aug 5 00:33:57 Vault kernel: bond0: (slave eth0): making interface the new active one
Aug 5 00:33:57 Vault kernel: device eth0 entered promiscuous mode
Aug 5 00:33:57 Vault kernel: bond0: active interface up!
Aug 5 00:33:57 Vault kernel: br0: port 1(bond0) entered blocking state
Aug 5 00:33:57 Vault kernel: br0: port 1(bond0) entered forwarding state
Aug 5 00:33:57 Vault dhcpcd[1121]: br0: carrier acquired
Aug 5 00:33:58 Vault dhcpcd[1121]: br0: rebinding lease of 192.168.1.232
Aug 5 00:34:00 Vault kernel: igc 0000:02:00.0 eth0: NIC Link is Down
Aug 5 00:34:00 Vault kernel: bond0: (slave eth0): link status definitely down, disabling slave
Aug 5 00:34:00 Vault kernel: device eth0 left promiscuous mode
Aug 5 00:34:00 Vault kernel: bond0: now running without any active interface!
Aug 5 00:34:00 Vault kernel: br0: port 1(bond0) entered disabled state
Aug 5 00:34:01 Vault dhcpcd[1121]: br0: carrier lost
Aug 5 00:34:05 Vault kernel: igc 0000:02:00.0 eth0: NIC Link is Up 1000 Mbps Full Duplex, Flow Control: RX/TX
Aug 5 00:34:05 Vault kernel: bond0: (slave eth0): link status definitely up, 1000 Mbps full duplex
Aug 5 00:34:05 Vault kernel: bond0: (slave eth0): making interface the new active one
Aug 5 00:34:05 Vault kernel: device eth0 entered promiscuous mode
Aug 5 00:34:05 Vault kernel: bond0: active interface up!
Aug 5 00:34:05 Vault kernel: br0: port 1(bond0) entered blocking state
Aug 5 00:34:05 Vault kernel: br0: port 1(bond0) entered forwarding state
Aug 5 00:34:05 Vault dhcpcd[1121]: br0: carrier acquired
Aug 5 00:34:05 Vault dhcpcd[1121]: br0: rebinding lease of 192.168.1.232
Aug 5 00:34:10 Vault dhcpcd[1121]: br0: probing for an IPv4LL address
Aug 5 00:34:10 Vault dhcpcd[1121]: br0: DHCP lease expired
Aug 5 00:34:10 Vault dhcpcd[1121]: br0: soliciting a DHCP lease
Aug 5 00:34:10 Vault dhcpcd[1121]: br0: offered 192.168.1.232 from 192.168.1.254
Aug 5 00:34:12 Vault dhcpcd[1121]: br0: probing address 192.168.1.232/24
Aug 5 00:34:15 Vault dhcpcd[1121]: br0: using IPv4LL address 169.254.25.30
Aug 5 00:34:15 Vault avahi-daemon[5897]: Joining mDNS multicast group on interface br0.IPv4 with address 169.254.25.30.
Aug 5 00:34:15 Vault avahi-daemon[5897]: New relevant interface br0.IPv4 for mDNS.
Aug 5 00:34:15 Vault avahi-daemon[5897]: Registering new address record for 169.254.25.30 on br0.IPv4.
Aug 5 00:34:15 Vault dhcpcd[1121]: br0: adding route to 169.254.0.0/16
Aug 5 00:34:15 Vault dhcpcd[1121]: br0: adding default route
Aug 5 00:34:15 Vault network: hook services: interface=br0, reason=IPV4LL, protocol=ipv4ll
Aug 5 00:34:15 Vault network: update services: 45s
Aug 5 00:34:17 Vault dhcpcd[1121]: br0: leased 192.168.1.232 for 86400 seconds
Aug 5 00:34:17 Vault avahi-daemon[5897]: Registering new address record for 192.168.1.232 on br0.IPv4.
Aug 5 00:34:17 Vault dhcpcd[1121]: br0: adding route to 192.168.1.0/24
Aug 5 00:34:17 Vault dhcpcd[1121]: br0: changing default route via 192.168.1.254
Aug 5 00:34:17 Vault dnsmasq[7969]: reading /etc/resolv.conf
Aug 5 00:34:17 Vault dnsmasq[7969]: using nameserver 192.168.1.254#53
Aug 5 00:34:17 Vault network: hook services: interface=br0, reason=BOUND, protocol=dhcp
Aug 5 00:34:17 Vault network: update services: 45s
Aug 5 00:34:17 Vault avahi-daemon[5897]: Withdrawing address record for 169.254.25.30 on br0.
Aug 5 00:34:17 Vault avahi-daemon[5897]: Leaving mDNS multicast group on interface br0.IPv4 with address 169.254.25.30.
Aug 5 00:34:17 Vault avahi-daemon[5897]: Joining mDNS multicast group on interface br0.IPv4 with address 192.168.1.232.
Aug 5 00:34:17 Vault dhcpcd[1121]: br0: deleting route to 169.254.0.0/16
Aug 5 00:34:17 Vault network: hook services: interface=br0, reason=IPV4LL, protocol=ipv4ll
Aug 5 00:34:17 Vault network: update services: 45s
Aug 5 00:34:19 Vault ntpd[1294]: Listen normally on 2 br0 192.168.1.232:123
Aug 5 00:34:19 Vault ntpd[1294]: new interface(s) found: waking up resolver
Aug 5 00:35:00 Vault network: reload service: nginx
Aug 5 00:58:53 Vault emhttpd: read SMART /dev/sdh
Aug 5 01:29:00 Vault emhttpd: spinning down /dev/sdh
Aug 5 02:05:07 Vault emhttpd: read SMART /dev/sdh
Aug 5 05:34:42 Vault emhttpd: spinning down /dev/sdh
Aug 5 11:53:26 Vault ool www[8441]: /usr/local/emhttp/plugins/dynamix/scripts/rsyslog_config
Aug 5 11:53:28 Vault rsyslogd: [origin software="rsyslogd" swVersion="8.2102.0" x-pid="8492" x-info="https://www.rsyslog.com"] start
Aug 5 12:55:59 Vault emhttpd: read SMART /dev/sdh
Aug 5 13:26:06 Vault emhttpd: spinning down /dev/sdh
Aug 5 14:27:55 Vault emhttpd: read SMART /dev/sdh
Aug 5 16:12:36 Vault emhttpd: spinning down /dev/sdh
Aug 5 18:10:46 Vault emhttpd: read SMART /dev/sdh
Aug 5 18:29:26 Vault kernel: nzbget[20589]: segfault at a ip 00000000004673bb sp 00001481a8f46c20 error 4 in nzbget[400000+5f7000] likely on CPU 3 (core 3, socket 0)
Aug 5 18:29:26 Vault kernel: Code: 30 48 8b 6f 28 48 89 74 24 28 48 39 c5 48 89 44 24 08 0f 84 66 01 00 00 0f 1f 80 00 00 00 00 48 8b 45 00 48 8b 98 38 03 00 00 <80> 7b 0a 00 0f 84 3b 01 00 00 4c 8b 2b 41 8b 85 3c 01 00 00 83 f8
Aug 5 19:04:31 Vault emhttpd: spinning down /dev/sdh
Aug 5 21:05:16 Vault emhttpd: read SMART /dev/sdh
Aug 5 23:31:11 Vault emhttpd: spinning down /dev/sdh