[ArgumentNullException: Value cannot be null.
Parameter name: source]
  System.Linq.Enumerable.SingleOrDefault(IEnumerable`1 source, Func`2 predicate) +467
  DotNetNuke.Entities.Portals.PortalAliasController.GetPortalAliasLookupInternal(String alias) +94
  DotNetNuke.Entities.Portals.PortalAliasController.GetPortalAliasInternal(String httpAlias) +31
  DotNetNuke.Entities.Portals.PortalAliasController.GetPortalAlias(String alias) +5
  DotNetNuke.Entities.Portals.PortalSettings.GetPortalAliasInfo(String portalAlias) +26
  DotNetNuke.Modules.ActiveForums.ForumsReWriter.OnBeginRequest(Object s, EventArgs e) +397
  System.Web.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +136
  System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +69

Cause:

Resolution:

1. Connect to your MSSQL database by following these instructions: http://support.managed.com/kb/a1833/how-to-connect-to-your-mssql-database.aspx
2. Once connected, open a new query by clicking 'New Query' and paste in the following query:

   •  USE [DATABASE]
     CREATE PROCEDURE dbo.GetPortalAliases
     
     AS
     
     SELECT * from dbo.PortalAlias

3. Replace "[DATABASE]" with the name of your database. If you are using a custom schema, or you have an object qualifier, be sure to make those changes accordingly. 
4. Execute the query by clicking 'Execute' or by pressing the 'F5' key.
5. Refresh the DNN upgrade page, and the DNN upgrade should run.