Due to performance issues and because server setting were not available i remove completely the Plex server from my NAS and installed latest version and now i got the error above. tried multiple times even delete all Plex folder with not luck. followed some recomendations from the forums with not progress on this issue.
Finally I got it working, and the problem was not the Plex Server. Few weeks ago I upgraded the memory on the Synology and when i installed the memory i pick the wrong size, instead of 2 Gb, i chose a 4 Gb , seriously by mistake, so the memory went up to 6 Gb instead of 4 Gb which is the maximum memory allowed memory on the DSS1513+ unit. As all was working fine i did not bother to replace it.
Eventually Plex started to experience lot of latency issues and I decided to removed and install all again and i ended in all sorts of issues.
I follow the recommendation provided by ChuckPA and same issue, so i said must be hardware issue and here we go. I replaced the proper memory and max it out as per 4 Gb spec an all started to work properly as it was all the time.
Thanks for the support ChuckPA and my story is good to be shared to the forum