OhLaLa,

not clear if it is the same error.  In this thread are 2 diffeent problems, that cased the same error message "A critical error has occurred. Vaue cannot be null...".  This error you will see on many issues also to other modules.  So the best is to look to the event log for the error logs and then check if this issue you have previuosly was handled in this forum.  If not create a new thread with your issue.

I understand you or your users have the problem that they can't no longer doing a post, since last posting.  The simple solution is then that the user goes in "my settings" of the Forum and subscript to all forums notification and saved the setting.  Perform a try to post.  If post running now, the user can go back to "my settings" and can unsubscript for all notifications.

The subject of this thread is not clear enouth to say it is the same issue.  Your issue is happend with the timeout and not with Listtable.  A Solution was posted several times for the time out issue in this Forum.  The most subject is some users can no longer add/edit post for this issue.  The solution is to set up the timeout in the web.config file from 20 minutes to 60 minutes.  In the next version the Forum will check on each post the timeout and then prevent for checking in.   This will have that all the text you have written is loosed, we can not have a other solution, about after timeout the user is no longer logged in in the portal.  So it is on you to setup the right timeout of the IIS.