Kodi thumbnails cache can grow over time and depending on your playing habits and media library collection can reach several gigabytes. Also Kodi does not automatically delete unused thumbnails. If you delete a video its cached artwork remains in the Thumbnails folder
Luckily, Kodi will re-cache needed artwork when required. So the easiest way to fix the low storage warning is to delete the userdata/Thumbnails folder and its accompanying database Database/TexturesXX.db. This function does just that.
If you're storing your media on a NAS or a large external drive, it's a good practice to export your library from time to time. In Settings:
system/media/library
Export Library
Separate
This will store a video's artwork and nfo file on the drive with the video. When Kodi needs to re-cache it will be almost instantaneous. This also makes rebuilding your library quite easy should some disaster occur.
If you have questions or comments, please email jocala@jocala.com or visit one of the forums linked below. adbLink produces a log file, adblink.log, with each run. Include it in your email or post if you're having problems.