Due to a sudden influx of spam, we've had to temporarily disable posting new messages to the forum. We will resolve the problem in some more satisfactory way as soon as we can - thanks for your patience!
As an update: existing users should now be able to post new discussion topics. New users (as of now) will be unable to post. We are still working on a suitable solution.
Posting by new users has been re-enabled, but we will disable it again if the spam flow resumes. Please report any spam to the usual support@greenfoot.org address. Thanks!