My Cloud EX2 Ultra The server is not powerful enough to convert video

Hi,
I am not sure how to start fixing this problem, I have been using Plex on my My Cloud EX2 Ultra NAS drive for a month and have had no issues until today when i try and watch videos from it I get an error message saying that the server is not powerful enough to convert the video. I have not changed any of the settings in the NAS or added any new videos. I am trying to watch videos that i have watched before on the NAS so have no idea why its doing this now. I have done all the usual rebooting it and the network but nothing has worked. This is my first ventures into a NAS drive and using Plex.