It seems that when you start to reply to a thread and get taken to a Login screen, that it is a different login screen as when you hit the Login button in the upper right. The one in the upper right works, the one when you hit reply to a thread when not logged in fails.
FYI.
It took me and Kaelten weeks to get the login as stable as it is now :(
In fact, I could edit it to leave an instructional message on how to save their post...