It was brought to my attention a short time ago that it appears that a few unverified users were unable to create new threads or post in existing threads.
It looks like this bug may have been caused by one of the site updates that rolled out this morning. I've notified robin about the issue, so hopefully he will be able to investigate and resolve it soon.
In the meantime, it looks like verified members who have already verified their accounts are unaffected. I'm not sure if this bug will disappear if you verify your account while this bug is present, so I'm not going to post that as a quick fix or stopgap measure to get around this bug unless someone can confirm that it works.
Also note: I'd like to stress that this was an unexpected software bug, and while we are currently encouraging members to verify their accounts, this was in no way intended to force anyone to do so. So, please bear with us while it gets sorted out. Thanks