I've tried to create a new discussion post from your discussion page, it seems work fine for me without hitting this error.
Perhaps I missing something how to reproduce this, can you provide us that user login account which hitting this issues so we can replicate it and resolve this quickly.
Tanks for the fix Arlex!
I have noticed something in the links discuss creates in the front page, is this normal?
When I click the link it goes the post but not the reply.. Check attached image for more info please.