I recently migrated all of my thumbnails in XBMC Eden to a single share on my Synology NAS but had all sorts of grief afterwards with random missing thumbnails. I finally got around to repairing the missing picture thumbs this evening and found that by simply renaming "\\xxx.xxx.xxx.xxx\Userdata\Database\Textures6.db" to "\\xxx.xxx.xxx.xxx\Userdata\Database\Textures6.old", this file is recreated and the picture thumbs will be regenerated. P.S I tried regenerating the thumbs using the option menu in XBMC and this did not work.
My collection of Home Assistant, CoreELEC, IoT, Synology, Kodi and other technical tips gathered over the years.