I am getting this exact issue as well for about the same time frame. Plex Media Player is running on the same PC as Plex Media Server. The first user to open PMP usually has no problems, but switching Plex accounts (usually from my account to the kids’ managed account) inside PMP often generates this error. It has not occurred in the web interface, or on other Windows PCs in the home running PMP.
Logging out and logging back in sometimes helps, restarting the server sometimes helps. It’s getting very frustrating!
Also, sometimes users get stuck on the “Discover” page, unable to browse libraries, but search still works to find something to watch.
