Hi,
I was able to produce this error locally by adding this to the theme.skin file
<asp:Menu Orientation="Horizonatal" />
Note that unless you are running in Medium Trust, it is using the theme.skin file from
/Data/Sites/[SiteID]/skins/[skinname]
in my test this was
/Data/Sites/1/skins/andreasvicklund-02
But the error will still say it is happening in
/App_Themes/Default/theme.skin
because the runtime thinks that is where it is getting the file from but a VirtualPathProvider is actually serving it from the
/Data/Sites/[SiteID]/skins/[skinname] folder.
When I remove the invalid <asp:Menu Orientation="Horizonatal" /> it fixes the problem right away
You really should not try to style the menu using theme.skin because we are using CSSControlAdapters tomake it render different than a standard ASP.NET menu which would render as ugly nested tables by default.
Hope it helps,
Joe