The Kunena team has announce the arrival of Kunena 7.0.4 [K 7.0.4] in stable which is now available for download as a native Joomla extension for J! 5.4.x/6.0.x. This version addresses most of the issues that were discovered in K 6.2 / K 6.3 / K 6.4 and issues discovered during the last development stages of K 7.0
I'm getting perhaps a related issue. When a user hits "Quick Reply" the subject field is not populated and the user is not accustomed to being asked for a new subject when hitting Quick Reply. this has only started since 5.1.1 That said even you user populates the new subject field, they still get the error:
"Cannot save message: Subject is empty!"
Using the Quote or Reply button and there isn't this problem. Just withthe Quick Reply