No, that would have been me as a registered user. Here is the same error as host though:
AssemblyVersion: 04.00.02
Method: DotNetNuke.Modules.Forum.ForumUserController.GetForumUser
FileName:
FileLineNumber: 0
FileColumnNumber: 0
PortalID: 0
PortalName: Brazos Area Homeschool Association
UserID: 1
UserName: host
ActiveTabID: 87
ActiveTabName: Forum
AbsoluteURL: /DotNetNuke/Default.aspx
AbsoluteURLReferrer: http://www.brazoshomeschoolers.org/DotNetNuke/Forum/tabid/87/Default.aspx
ExceptionGUID: 4993ff19-1072-47db-ba07-946f9565eac0
DefaultDataProvider: DotNetNuke.Data.SqlDataProvider, DotNetNuke.SqlDataProvider
InnerException: Object reference not set to an instance of an object.
Message: System.NullReferenceException: Object reference not set to an instance of an object. at DotNetNuke.Modules.Forum.ForumUserController.GetForumUser(Int32 UserID, Boolean UpdateLastVisit)
StackTrace:
Source:
Server Name: WINHOST22
Do you think it could possibly be trying to find a user that was deleted, but before being deleted had posted several times?
I am so glad you have figured out why the email notification was working sporadically! I wish I knew enough about programing to help 
I am a real newbie here, but I am learning!
The version we are currently using is GoDaddy's value application one, but my small team and I are going to work the next month trying to figure out some of the problems with this one before we load our own copy. GoDaddy refuses to let us have a backup copy, so we have to start from scratch. Here is the web addy for the site if you want to take a look. www.brazoshomeschoolers.org/dotnetnuke
Thanks for all you are doing, as a volunteer trying to develop a great website for our organization off of your labors, I know it can be a thankless job sometimes.
Michelle