I also thought of posting this problem, looks like you did first. Yes, I am facing the same issue. The preview button simply don't work at certain times. If I try to preview, it will show a message saying "The message body was left empty", even though the body is full of contents. Weird bug.
Note: I'm using Chrome and always connected to VPN when accessing the internet.
imageuploadThis happens when there is an emoji in the text of a message or in its title, something like 👓 ✏️. When you go to the preview page it won't work, but if you will tap a Post button, the message will be posted without any problems. If you don't want to see this error, you can remove all emojis from the text and the title.
I tested it. Emoji or no emoji, if it's inside the body, the preview button still works. But if I put the emoji inside the title/subject this error occurs. Note that even after reverting back to original title the error remains.