The issue should now be resolved. The root cause was that some new users had not provided a Private Email Address when accessing the site for the first time. The forum software requires that every user have an email address. We set any blank email addresses to "replaceme@test.com" and this enabled the forum software to function properly again.
Having a valid email address on your user account is important in case you wish to enable email subscriptions to any of the forums on this site. You can set your Private Email Address by clicking on your username in the upper right hand corner, select the Email tab, and then providing a valid email address for "Private Email Address", the last field on the form.
New users accessing the site for the first time are prompted to provide a Private Email Address but are not forcibly required to do so. So a number of people have bypassed that prompt. We will work on a better process for new users so that the issue does not reoccur.