I’ve never run Plex on a small NAS like that for any length of time, but have you already gone through the information in these links and checked to see if they’re related or solve the problem? I’m not sure if this is the time period you’re referring to, but was just searching for previous threads related to issues on My Cloud devices.
Plex can’t connect securely to WD My Cloud - Plex Media Server - Plex Forum
Is Plex still supported by My Book? - External Drives / My Book - WD Community
My prior experience trying to run Plex on a small consumer NAS was always poor for a variety of reasons, but if all your content will direct play, I assume it should be usable with a reasonable number of concurrent streams.
