Thursday I was copying and pasting comments from my old Weebly blog to new wp site. I pasted 20 successfully. On the 21st I suddenly got a 404 error on wp-comments-post.php. Depending on the browser, it's either 'page not found' or 'search returned no result'. I DID NOTHING to change anything. I was happily reposting comments and this appeared.
Searched forums, dealt with permalinks, cache, plugins, db table auto-increment. NOTHING. Finally I went to web host. Tier 1 could post (I can see post needs moderation). Sent to tier 2, then tier 3. They can all post comments and think I'm crazy.
I tried 3 different machines in my office. Had a friend try, my husband and sons from their laptops. The only people who can post are support at my host, or me, if I am logged in as admin. Everyone else gets a 404 error.