That specific container, published by plex themselves, doesn't have a support area on this forum, instead the plex authors support it on their own forums. https://forums.plex.tv/
There are other plex containers in the app section of Unraid that ARE supported here, and have their own support threads.
https://forums.unraid.net/topic/44141-support-binhex-plex/
Is an example of a container supported on these forums.
If you can't find the answers you need about the plex "official" container on their forums, maybe try installing one of the Unraid specific containers and see if that works for you.
I don't have any good input on your specific issue, I personally don't use plex for a few different reasons, I prefer emby, but that's a whole 'nother discussion.
The reason we need people to keep support requests for containers in the specific areas and threads designated is so the people most likely to be able to solve the problem don't have to keep track of posts on the whole forum, rather they can concentrate on their areas of expertise.