Hi. I am having a very similar issue.
site address is https://www.uncleartmusic.com
WP was in a different folder originally, however I’m pretty sure by the time I had installed jetpack I had migrated it over to the root folder.
Normal comments work fine without jetpack.
However when I click submit, if I’m logged in as admin. Its brings up, what looks like my homepage. If I’m not logged in, its brings up 404.
Any help would be much appreciated, as I really do prefer the jetpack comments!
