I get the 500 also. If you think about it, what is happening is that the reply id does not exist until you submit it, then before it is posted to the DB to update the table that id is called on and a fresh page is loaded. You get the 500 because this is what we call a "race condition".
That means that the server has not updated the database before the same data is requested.
If you notice once you use the back key the post is there.
This tells me that its possible that during last weeks downtime, someone may have:
1. Made some changes to the code and causing the issue.
2. Made some changes to the server config.
3. Changed servers and its not fully configed yet.
I hope that helps...
Also i believe the oxwall mail server is either down or broken as i have not received any activity notices since last week.