I have this issue, too.  It looks like a bug to me.

In TabController.vb, around line 1270 in SerializeTab()... there is a line that operates on hTabs(objTab.ParentId).  However, if the parent tab hasn't been processed yet (if it hasn't been added to the hTabs hashtable), then is throws a NullRefException.  I changed the source code locally to look like this below, and it fixes the EXPORT.  It's not efficient (but who cares for this feature, really), and I didn't put any thought into a better solution - but this fixes the EXPORT issue.  But, this isn't the complete solution - while it does solve the EXPORT problem, on IMPORT, it didn't create my ADMIN tab on the new site when I created it from this template.  So, there's more to it...but the root of the evil is somewhere in TabController.vb around line 1270-1280 where it operates on hTabs(objTab.ParentId).  I will try to see why it didn't create my Admin menu. 

 'Manage Parent Tab
            If hTabs IsNot Nothing Then
                If Not Null.IsNull(objTab.ParentId) Then
                    newnode = xmlTab.CreateElement("parent")
                    Dim tabName As String = ""
                    If hTabs(objTab.ParentId) Is Nothing Then
                        Dim tabs As List(Of TabInfo)
                        tabs = TabController.GetPortalTabs(objPortal.PortalID, -999, True, True)
                        For Each t As TabInfo In tabs
                            If t.TabID = objTab.ParentId Then
                                tabName = t.TabName
                                Exit For
                            End If
                        Next
                    Else
                        tabName = HttpContext.Current.Server.HtmlEncode(hTabs(objTab.ParentId).ToString)
                    End If
                    newnode.InnerXml = tabName
                    nodeTab.AppendChild(newnode)

                    ' save tab as: ParentTabName/CurrentTabName
                    hTabs.Add(objTab.TabID, tabName + "/" + objTab.TabName)
                Else
                    ' save tab as: CurrentTabName
                    hTabs.Add(objTab.TabID, objTab.TabName)
                End If
            End If

Great - Dan, can you now log this at support.dotnetnuke.com?

Many thanks.

Just to close this out... Charles did some research on this and concluded that the only change necessary is the fix for the "<10000" - and I concur. It was because of this bug that the Admin tab didn't get processed first, and so when the Extensions tab was to be processed in the export, it bombed because the Admin tab hadn't been added to the collection yet because its TabOrder was > 10000.

If you need a QUICK fix for this solution, here is what I did...I just ran this on my database and that fixed it.  It just brings the tabOrder for the admin tabs down below 10000.

UPDATE TAbs
SET TabOrder = TabOrder - 5000
WHERE TabOrder >= 5000

Charles did make the bug fix and it sounds like it will be in the next release.

Dan