We came across this particular text on our forum which exhibit the problem in question: 🇺🇸
In the full post editor, clicking the "Preview" button will give a "message body was left empty" error and won't preview. However, clicking the "Post" button (or "Save" button if editing an existing post) works without error. It's just the preview which fails.
This problem occurs here on the SMF forum as well, indicating it's a bug in SMF 2.0.x. You can easily reproduce it by simply quoting this post and then trying to preview it.
Yes, this is related to the utf8mb4 fix which was added to 2.0.10 but it seems to have missed the preview.
Before 2.0.10 it would just crop your post at the symbol...
This was reported a few times & a fix is targeted for 2.0.16 - I am going to close some dupes & keep one open:
https://www.simplemachines.org/community/index.php?topic=569620.0