I have a course on a 2.3 site that was installed from a 1.9 version. The assignments were updated using the upgrade tool and even though the assignments are set up to accept online text and allow for comments the assignments tell the students that a submission is not required.

To fix this I have gone in to edit the assignment settings, change online text to "no" then save, edit, change back to "yes" and then it works again.
The same fix works for the comments.