There are is another thread concerning this issue as well that can be found here: Plex Player stopped working on MacPro 2013 - #25 by iamdinux - I also have another thread here: Mac Player Cannot Establish Connection to Server(s) - #8 by iamdinux - I think all of the issues are linked and have to do specifically with the latest update to the client player. We are waiting to hear back from support though. I reached out via the website contact form to billing because we have not gotten a timely response regarding our problems and it looks like more and more users are reporting the same issues.
By chance, are you on a MacPro 6,1 (2013) Trashcan?
