Hello,
I see error in my log and need help to resolved that 
Server:
* DSM 6.1.4-15217 Update 2
* Plex Server 1.10.0.4523-648bc61d4 (Intel)
Client
* Windows 10 Pro
* Google Chrome 63.0.3239.84
IP V4 Static with 1500 MTU.
IP V6 disabled on Plex Server and Synology NAS.
Edit:
I updated to Plex 1.10.1.4561-9de2951fc, same error no fix.
I upload new Logs.
ERROR in my log
DEBUG - HTTP requesting POST http://127.0.0.1:32600/devices/discover
ERROR - Error issuing curl_easy_perform(handle): 7
WARN - HTTP error requesting POST http://127.0.0.1:32600/devices/discover (0, No error) (Failed connect to 127.0.0.1:32600; Connection refused)
ERROR - Error parsing content.
DEBUG - NetworkService: Network change for browser (polled=0), closing 0 browse sockets.
DEBUG - NetworkService: Setting up multicast listener on 0.0.0.0:1901 (outbound: 0)
ERROR - Error parsing XML.
DEBUG - Reading 486 bytes in the body, code is 200
DEBUG - Transcoder: Cleaning old transcode directories.
ERROR - Transcoder: Error cleaning old transcode sessions: boost::filesystem::directory_iterator::construct: No such file or directory: "/volume1/Plex/Library/Application Support/Plex Media Server/Cache/Transcode/Sessions"
ERROR - Network Service: Error in browser handle read: 125 (Operation canceled) socket=-1
Today, it's impossible to connect to my Plex Server with my local IP (http://dsm_server:32400/web) but it's ok with https://plex.tv and Launch Button.
Thanks by advance for your help.