Your suggestion to run sudo yunohost tools basic-space-cleanup shaved off almost 20GiB, so that’s a help. And thanks for recommending ncdu. It tells me that the big hog is /home/yunohost.app/lemmy/magick, which contains 44,259 files taking 405GiB in total. These files don’t have extensions or discernible filenames; “magick-o6aN5V0h1tB0T_WDGtaIDxgTIeuP0CbC” is one example. I have no idea why they are here taking up so much space. Any ideas?