Error: Value cannot be null. Parameter name: value DotNetNuke Forum Module.

Applies to:
DotNetNuke version 4.x.x

Symptoms:
When you click on a forum you get the following error:

Exception Details: System.ArgumentNullException: Value cannot be null.
Parameter name: value

Message: DotNetNuke.Services.Exceptions.PageLoadException: Value cannot be null. Parameter name: value ---> System.ArgumentNullException: Value cannot be null. Parameter name: value at System.Web.Caching.CacheEntry

Cause:
After a while of inactivity the user gets logged out, which causes the "AuthorID" to default to 0 in the database, and since there is no user with a UserID of 0 it causes a 'value cannot be null" exception.

Work Around:
To fix this in the database, there are two tables that needs to have their entries modified. they are "Forum_Forums" and "Forum_Posts"

1. In "Forum_Forums" table, find the entry for the affected forum. Within that entry is a field called "MostRecentPostAuthorID" you will notice that this ID is 0, change that to a valid UserID
2. In "Forum_Posts" find the corresponding entry within that table and change the UserID to a valid UserID.  The same UserID as above. (it will have a UserID of 0)