Thanks for the bug report, this is now fixed in the source code repository and on the demo site and this site.
For anyone who needs to fix it on their own site, use a text editor to edit the file /Forums/Controls/ForumList.ascx
Look for the 2 elements id="tdSubscribed" and id="tdSubscribedAlt"
enableviewstate='<%# ShowSubscribeCheckboxes %>'
note that the use of single quotes is important