Upgrade of a 3.5.6 site with two child portals to 4.4.0 resulted in "Server Error in / Application" when hitting the site the first time after an apparently successful install. After editing web.config to set customErrors = "Off", I was able to see the true error - "Subquery returned more than 1 value . . . " which originated in the GetPortal stored proceedure. The error was further traced to a subquery in the (new in 4.4) database view vw_Portals that is supposed to return the SuperTabId and ultimately to the Tabs table which had two rows where the PortalID and the ParentID were both null. The incorrect row was the one with TabName = Lists having a null in the ParentId column rather than (in my case at least) a 7 to point back to the SuperTabId. Restoring the ParentId column of this row to 7 corrected the problem.

I then looked at the database backup I had made just before the upgrade and found that the ParentId for the Lists tab in the Tabs table was null - so the error had been introduced at some time prior to the 4.4 upgrade and possibly sometime since the upgrade to 3.5.6 about a month ago.  That upgrade did not cause a similar problem. Although the GetPortal sproc has changed significantly between these versions to use the new vw_Portals view, it appears that the same subquery to obtain the SuperTabId was used in both versions.

As I've had not occasion to use the Lists tab for many months, the error in the Tabs table might have been lying dormant for many months.

Hope this will help someone with a similar proble!

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Bill, WESNet Designs
Team Lead - DotNetNuke Gallery Module Project (Not Actively Being Developed)
Extensions Forge Projects . . .
Current: UserExport, ContentDeJour, ePrayer, DNN NewsTicker, By Invitation
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I got the same error, there is a fix (needs some SQL)

Simply doing the following SQL
SELECT * FROM Tabs
WHERE (PortalID IS NULL) AND (ParentId IS NULL)
returns many tabs. You need to look for a tab with TabName='Host' and note its ID (in my case it was 7)
For the other returned rows use the previous obtained ID in the ParentId.

Kill the aspnet process and browse the site again, things are working fine. 

I have the same problem, when update to 4.4.0
but when I excute this query 2 rows whith NULL value in ParentID...