Below the log. We clearly see that the plex code is validating the url in request Request came in with unrecognized domain / IP 'plex.mydomain.com' in header Host; treating as non-local and that the request comes from the allowed network Request: [192.168.2.52:50485 (Allowed Network)] . To me it looks like a bug. The request comes from allowed network it should connect as expected.
Jul 01, 2018 21:28:29.405 [0x7fc1d83ff700] DEBUG - Request came in with unrecognized domain / IP 'plex.mydomain.com' in header Host; treating as non-local
Jul 01, 2018 21:28:29.407 [0x7fc1c5fff700] DEBUG - Request: [192.168.2.52:50485 (Allowed Network)] GET /web/index.html (3 live) TLS GZIP Signed-in
Jul 01, 2018 21:28:29.407 [0x7fc1c5fff700] DEBUG - Final path: "/volume1/.@plugins/AppCentral/plexmediaserver/plexmediaserver/Resources/Plug-ins-fd05be322/WebClient.bundle/Contents/Resources/index.html"
Jul 01, 2018 21:28:29.408 [0x7fc1c5fff700] DEBUG - Content-Length of /volume1/.@plugins/AppCentral/plexmediaserver/plexmediaserver/Resources/Plug-ins-fd05be322/WebClient.bundle/Contents/Resources/index.html is 9464.
Jul 01, 2018 21:28:29.410 [0x7fc1d7bfe700] DEBUG - Completed: [192.168.2.52:50485] 200 GET /web/index.html (3 live) TLS GZIP 3ms 9464 bytes (pipelined: 1)
Jul 01, 2018 21:28:29.426 [0x7fc1d83ff700] DEBUG - Request came in with unrecognized domain / IP 'plex.mydomain.com' in header Host; treating as non-local
Jul 01, 2018 21:28:29.428 [0x7fc1d4bff700] DEBUG - Request: [192.168.2.52:50485 (Allowed Network)] GET /web/main.36fc225172532dbfc10f4cbe8c9ecd1a.css (6 live) TLS GZIP Signed-in
Jul 01, 2018 21:28:29.429 [0x7fc1d4bff700] DEBUG - Final path: "/volume1/.@plugins/AppCentral/plexmediaserver/plexmediaserver/Resources/Plug-ins-fd05be322/WebClient.bundle/Contents/Resources/main.36fc225172532dbfc10f4cbe8c9ecd1a.css"
Jul 01, 2018 21:28:29.430 [0x7fc1d4bff700] DEBUG - Content-Length of /volume1/.@plugins/AppCentral/plexmediaserver/plexmediaserver/Resources/Plug-ins-fd05be322/WebClient.bundle/Contents/Resources/main.36fc225172532dbfc10f4cbe8c9ecd1a.css is 764347.
Jul 01, 2018 21:28:29.459 [0x7fc1d7bfe700] DEBUG - Request came in with unrecognized domain / IP 'plex.mydomain.com' in header Host; treating as non-local
Jul 01, 2018 21:28:29.460 [0x7fc1d83ff700] DEBUG - Request came in with unrecognized domain / IP 'plex.mydomain.com' in header Host; treating as non-local
Jul 01, 2018 21:28:29.461 [0x7fc1c5fff700] DEBUG - Request: [192.168.2.52:50489 (Allowed Network)] GET /web/main.v3.190ddbb2ebc8cbb2599e6f8fb82e7192.css (6 live) TLS GZIP Signed-in
Jul 01, 2018 21:28:29.462 [0x7fc1c5fff700] DEBUG - Final path: "/volume1/.@plugins/AppCentral/plexmediaserver/plexmediaserver/Resources/Plug-ins-fd05be322/WebClient.bundle/Contents/Resources/main.v3.190ddbb2ebc8cbb2599e6f8fb82e7192.css"
Jul 01, 2018 21:28:29.462 [0x7fc1c5fff700] DEBUG - Content-Length of /volume1/.@plugins/AppCentral/plexmediaserver/plexmediaserver/Resources/Plug-ins-fd05be322/WebClient.bundle/Contents/Resources/main.v3.190ddbb2ebc8cbb2599e6f8fb82e7192.css is 253644.
Jul 01, 2018 21:28:29.462 [0x7fc1d4bff700] DEBUG - Request: [192.168.2.52:50490 (Allowed Network)] GET /web/js/vendors-2-1b13c910b4c369fdbadf-plex-3.53.7-1c80782.js (6 live) TLS GZIP Signed-in
Jul 01, 2018 21:28:29.463 [0x7fc1d83ff700] DEBUG - Request came in with unrecognized domain / IP 'plex.mydomain.com' in header Host; treating as non-local
Jul 01, 2018 21:28:29.463 [0x7fc1d4bff700] DEBUG - Final path: "/volume1/.@plugins/AppCentral/plexmediaserver/plexmediaserver/Resources/Plug-ins-fd05be322/WebClient.bundle/Contents/Resources/js/vendors-2-1b13c910b4c369fdbadf-plex-3.53.7-1c80782.js"
Jul 01, 2018 21:28:29.464 [0x7fc1d4bff700] DEBUG - Content-Length of /volume1/.@plugins/AppCentral/plexmediaserver/plexmediaserver/Resources/Plug-ins-fd05be322/WebClient.bundle/Contents/Resources/js/vendors-2-1b13c910b4c369fdbadf-plex-3.53.7-1c80782.js is 341088.
Jul 01, 2018 21:28:29.465 [0x7fc1c5fff700] DEBUG - Request: [192.168.2.52:50491 (Allowed Network)] GET /web/js/main-1-20c05ad369662168ab2a-plex-3.53.7-1c80782.js (6 live) TLS GZIP Signed-in
Jul 01, 2018 21:28:29.466 [0x7fc1c5fff700] DEBUG - Final path: "/volume1/.@plugins/AppCentral/plexmediaserver/plexmediaserver/Resources/Plug-ins-fd05be322/WebClient.bundle/Contents/Resources/js/main-1-20c05ad369662168ab2a-plex-3.53.7-1c80782.js"
Jul 01, 2018 21:28:29.466 [0x7fc1c5fff700] DEBUG - Content-Length of /volume1/.@plugins/AppCentral/plexmediaserver/plexmediaserver/Resources/Plug-ins-fd05be322/WebClient.bundle/Contents/Resources/js/main-1-20c05ad369662168ab2a-plex-3.53.7-1c80782.js is 3691822.
Jul 01, 2018 21:28:29.477 [0x7fc1d83ff700] DEBUG - Completed: [192.168.2.52:50485] 200 GET /web/main.36fc225172532dbfc10f4cbe8c9ecd1a.css (6 live) TLS GZIP 48ms 764347 bytes (pipelined: 2)
Jul 01, 2018 21:28:29.479 [0x7fc1d83ff700] DEBUG - Completed: [192.168.2.52:50489] 200 GET /web/main.v3.190ddbb2ebc8cbb2599e6f8fb82e7192.css (6 live) TLS GZIP 17ms 253644 bytes (pipelined: 1)
Jul 01, 2018 21:28:29.483 [0x7fc1d83ff700] DEBUG - Completed: [192.168.2.52:50490] 200 GET /web/js/vendors-2-1b13c910b4c369fdbadf-plex-3.53.7-1c80782.js (6 live) TLS GZIP 21ms 341088 bytes (pipelined: 1)
Jul 01, 2018 21:28:29.658 [0x7fc1d83ff700] DEBUG - Completed: [192.168.2.52:50491] 200 GET /web/js/main-1-20c05ad369662168ab2a-plex-3.53.7-1c80782.js (6 live) TLS GZIP 193ms 3691822 bytes (pipelined: 1)
Jul 01, 2018 21:28:30.375 [0x7fc1d7bfe700] DEBUG - Request came in with unrecognized domain / IP 'plex.mydomain.com' in header Host; treating as non-local
Jul 01, 2018 21:28:30.375 [0x7fc1d83ff700] DEBUG - Request came in with unrecognized domain / IP 'plex.mydomain.com' in header Host; treating as non-local
Jul 01, 2018 21:28:30.376 [0x7fc1d4bff700] DEBUG - Request: [192.168.2.52:50491 (Allowed Network)] GET /web/translations/en.json (6 live) TLS GZIP Signed-in
Jul 01, 2018 21:28:30.377 [0x7fc1d7bfe700] DEBUG - Request came in with unrecognized domain / IP 'plex.mydomain.com' in header Host; treating as non-local
Jul 01, 2018 21:28:30.377 [0x7fc1d4bff700] DEBUG - Final path: "/volume1/.@plugins/AppCentral/plexmediaserver/plexmediaserver/Resources/Plug-ins-fd05be322/WebClient.bundle/Contents/Resources/translations/en.json"
Jul 01, 2018 21:28:30.378 [0x7fc1d4bff700] DEBUG - Content-Length of /volume1/.@plugins/AppCentral/plexmediaserver/plexmediaserver/Resources/Plug-ins-fd05be322/WebClient.bundle/Contents/Resources/translations/en.json is 2.
Jul 01, 2018 21:28:30.379 [0x7fc1c5fff700] DEBUG - Request: [192.168.2.52:50485 (Allowed Network)] GET /web/common/img/backgrounds/preset-dark.64cc1c942221cd2c153244bd8ecfb67a.png (6 live) TLS GZIP Signed-in
Jul 01, 2018 21:28:30.380 [0x7fc1c5fff700] DEBUG - Final path: "/volume1/.@plugins/AppCentral/plexmediaserver/plexmediaserver/Resources/Plug-ins-fd05be322/WebClient.bundle/Contents/Resources/common/img/backgrounds/preset-dark.64cc1c942221cd2c153244bd8ecfb67a.png"
Jul 01, 2018 21:28:30.380 [0x7fc1d7bfe700] DEBUG - Completed: [192.168.2.52:50491] 200 GET /web/translations/en.json (6 live) TLS GZIP 4ms 2 bytes (pipelined: 2)
Jul 01, 2018 21:28:30.381 [0x7fc1c5fff700] DEBUG - Content-Length of /volume1/.@plugins/AppCentral/plexmediaserver/plexmediaserver/Resources/Plug-ins-fd05be322/WebClient.bundle/Contents/Resources/common/img/backgrounds/preset-dark.64cc1c942221cd2c153244bd8ecfb67a.png is 31365.
Jul 01, 2018 21:28:30.382 [0x7fc1ca7ff700] DEBUG - Request: [192.168.2.52:50490 (Allowed Network)] GET /web/common/img/backgrounds/noise.8b05ce45d0df59343e206bc9ae78d85d.png (6 live) TLS GZIP Signed-in
Jul 01, 2018 21:28:30.383 [0x7fc1ca7ff700] DEBUG - Final path: "/volume1/.@plugins/AppCentral/plexmediaserver/plexmediaserver/Resources/Plug-ins-fd05be322/WebClient.bundle/Contents/Resources/common/img/backgrounds/noise.8b05ce45d0df59343e206bc9ae78d85d.png"
Jul 01, 2018 21:28:30.384 [0x7fc1d7bfe700] DEBUG - Completed: [192.168.2.52:50485] 200 GET /web/common/img/backgrounds/preset-dark.64cc1c942221cd2c153244bd8ecfb67a.png (6 live) TLS GZIP 4ms 31365 bytes (pipelined: 3)
Jul 01, 2018 21:28:30.384 [0x7fc1ca7ff700] DEBUG - Content-Length of /volume1/.@plugins/AppCentral/plexmediaserver/plexmediaserver/Resources/Plug-ins-fd05be322/WebClient.bundle/Contents/Resources/common/img/backgrounds/noise.8b05ce45d0df59343e206bc9ae78d85d.png is 54413.
Jul 01, 2018 21:28:30.387 [0x7fc1d83ff700] DEBUG - Completed: [192.168.2.52:50490] 200 GET /web/common/img/backgrounds/noise.8b05ce45d0df59343e206bc9ae78d85d.png (6 live) TLS GZIP 7ms 54413 bytes (pipelined: 2)
Jul 01, 2018 21:28:30.517 [0x7fc1d7bfe700] DEBUG - Request came in with unrecognized domain / IP 'plex.mydomain.com' in header Host; treating as non-local
Jul 01, 2018 21:28:30.517 [0x7fc1d7bfe700] DEBUG - Request: [192.168.2.52:50491 (Allowed Network)] GET / (6 live) TLS GZIP Signed-in
Jul 01, 2018 21:28:30.519 [0x7fc1d7bfe700] DEBUG - Completed: [192.168.2.52:50491] 401 GET / (6 live) TLS GZIP 1ms 416 bytes (pipelined: 3)
Jul 01, 2018 21:28:30.569 [0x7fc1d83ff700] DEBUG - Request came in with unrecognized domain / IP 'plex.mydomain.com' in header Host; treating as non-local
Jul 01, 2018 21:28:30.570 [0x7fc1c5fff700] DEBUG - Request: [192.168.2.52:50491 (Allowed Network)] GET /identity (6 live) TLS GZIP Signed-in
Jul 01, 2018 21:28:30.574 [0x7fc1d83ff700] DEBUG - Completed: [192.168.2.52:50491] 200 GET /identity (6 live) TLS GZIP 3ms 471 bytes (pipelined: 4)