I am really struggling with this. I followed all your steps and some more from here and here. I can’t seem to get any previews to work. This has been a problem ever since my server updated to NC22.
My oc_filecache database is 165MB in size but when executing delete from oc_filecache where path LIKE 'appdata_%/preview/%'; zero rows are affected.
Edit: By replacing % with * it still says zero rows affected but the table shrunk to <60MB. After generating all previews again it still does not work, sadly.