Blog posts keep failing to post + lose all progress

Any time I make a blog post longer than a few paragraphs or so, especially if I add photos, the post gets deleted as I post it, linking to https://blog.spacehey.com/entry?id=0 which is a nonexistent post. Hitting the "Keep editing" button won't save it either, as all the progress gets deleted as well. Does anyone know what's causing this specifically and how I can stop it from happening? TIA