I received this same message following a test upgrade of a staging site from 4.05.05 to 4.08.00 that seemed to go fine until I clicked on the link to visit the upgraded portal (single portal installation). I also received it during a fresh install of a 4.08.00 site on my development box which was interupted during the install when my dog yanked on the computer's power cord. [Working from home has its dangers!]
Here's a theory as to what is happening:
As part of the 4.07.00 upgrade, a new section is added to the file DotNetNuke.config. This section specifies the default skin with is to be loaded if no other is specified or a non-default skin is not found for some reason. Should the upgrade not complete (for some, perhaps unrelated reason) or be interrupted by dogs or other natural causes, the new section never gets added to DotNetNuke.config. Without the specification of the default skin, a fresh install, an upgrade of a site which previously used the default skin, or (perhaps) the creation of a new child portal will error in this manner.
When I inserted the "skinningdefaults" node just before the closing </configuration> tag in the DotNetNuke.config file, the error resolved itself and both sites became accessible and seemed to function properly. Becasue of the likelyhood of other problems I chose to rollback the first site and recreate the second. The "skinningdefaults" node is as follows:
<skinningdefaults>
<skininfo admindefault="Horizontal Menu - Fixed Width.ascx" default="Horizontal Menu - Fixed Width.ascx" folder="/DNN-Blue/">
</skininfo>
<containerinfo admindefault="Image Header - Color Background.ascx" default="Image Header - Color Background.ascx" folder="/DNN-Blue/">
</containerinfo>
</skinningdefaults>
Rather than copying and pasting from the above which contains escaped characters, look for the "skinningdefaults" section in the file 04.07.00.config in the /portals/_default/ folder.