Some updates back, I started receiving this type of error. Plex is useless now. I am attaching the server logs. I am running Plex on a Windows 8.1 system.
I have spent few months updating PLEX and getting codec errors when trying to watch a movie. Finally, I decided to copy the codec from other sources into the codec folder and finally, I was able to watch some movies on my TV. So, probably that error is related to the copy / paste process.
Sorry. I should have been more explicit. I need the logs after you get the error message, so I can look why. I don’t see that error showing up in your logs. Your copy/pasting of the codecs could now be masking the problem. Can you temporarily remove all the codec files, try to play something, then after you see the error get the logs again. You can put the codec files back after so you can resume watching.
If there are lots of repeated attempts to download the files you will get rate limited and downloads would fail. The error that we would need to see is the first error before any rate limiting. If you do get rate limited, wait an hour before retrying and then capture the logs when the codecs fail to download