Per https://www.phpbb.com/community/viewtopic.php?f=556&t=2512406 a banned user (2 week ban) is reporting when they try to login they receive the "The submitted form was invalid. Try submitting again." message. ACP->Security settings->Tie forms to guest sessions is set to "Yes". 

If ACP->Security settings->Tie forms to guest sessions is set to "No" then the proper and expected message displaying the banned reason. 

This problem can be reproduced on a Quickinstall board using stock phpBB 3.2.7 as detailed in the post https://www.phpbb.com/community/viewtopic.php?p=15261816#p15261816