I just tried to post a reply to a community discussion, and I got an error that say something like "There was an error posting the reply. Please try again." So I tried again, and got the same thing. And again. So I logged out of the community forums (which did bring up the page giving me the option of logging out of SSO), but trying to log back into the community forums just sent me back to the page with the option to log out of SSO. So I logged out of SSO, then logged back in (using SSO), and indeed all of my post requests had succeeded and I had multiple copies of my reply in the thread.

Incidentally, this morning I received two email notifications of posts on the same thread, where each post had the same content but had different URLs. One of the URLs worked, while the other one did not. I suspect the user experienced the same thing as me.