This has happened to me 4 times in the past year, and 2 times it happened last night around 3AM so I wasn't exactly on top of my game, but I did remember to get diagnostics both times.   I was watching plex struggle with a music folder of bad .wav files, and determined the rip was bad so I went to delete the folder via NFS and Linux Mint 21 reported "Can not delete folder, not empty." I went to look at the folder and all the contents were deleted, it was empty so I deleted the folder t