@jamesrayth Hmmm....this is a bit messy. I suspect that what you're encountering is both versioning mismatch AND, as a result, maybe permissions as well. I typically do not recommend attempting to configure Plex via TN particularly as Plex is a curated and a fully supported app. Doing that can easily create conflicts (this has happened to me) and can lead to exactly the kind of permissions drift I think is occurring. The error you’re getting suggests HexOS no longer has permission to write to its app storage location.
Wit that said, maybe the safest approach would be to use TN UI to repair permissions on the Applications dataset/folder ,making sure the application has write access and then go back to HexOS to install Plex normally. Hopefully, once HexOS can write to the apps a Plex re-install should behave again.
PLEASE make sure you're data is backed up and good luck!!