I’m resolved (but I’m not the OP). I saw this thread, https://ww.wp.xz.cn/support/topic/404-error-publishing-post-with-html/ and I’m hosted with Namecheap, so I contacted them.
Took about 15 minutes, but it was resolved as a false positive modsecurity on their side.
I too just started experiencing this. I noticed it only happens if I have html-like character in the post/page. Things like < or & cause the issue. Removing that, and it will save/preview/etc like it should.
Are you seeing that too?
