Necro-ing the thread for anyone who may have the same issue. I had this issue recently running my Plex on a Synology NAS. I created a new library, but no matter what I did I kept getting the no files found message. Meanwhile I had no issue placing movies into my other libraries. I eventually found it under properties for the folder on the NAS itself. When I created the folder, it only gave permissions to me, the admin. I had to go in to properties, and change the plex permissions from the default deny read/write to allow read/write. After that, I had no further problems.
