make sure your media is available.
for example, my nas overheated and shutdown overnight and i couldn’t resume watching a title getting this error.
checking my server, an imac, shares had disappeared from the desktop. i restarted my beleaguered nas, reconnected to the shares. problem solved
i would guess that if your media was on server on any removable device and the media is removed or powered off, you get this error. and of course, this would only occur after you scanned a library and created the link to files on the media. this error might also occur if you rename a directory in a path, etc.
happy plexing!