500 Internal Error When Replying to Posts

For a couple of years, whenever anyone tries to reply/comment on a post, a 500 internal error is the result.

I have had debug on and no errors are logged. I have turned plugins on and off and nothing changes. I have tried a different theme and nothing changes.

I have also posted at https://wordpress.org/support/topic/500-error-when-replying-to-comment/ and they said to try here.

Please help

Thanks