Wow, this is a thread that hasn't been touched in awhile!
This error occurs when .RESX and .resources compilation, disabled by DotNetNuke is restored (either through web.config changes, or, less frequently, with cascading web.config files). No error will manifest until a portal-specific localization is performed, after which ASP.NET will unhelpfully report the error listed above.
For those that truly require ASP.NET localization functionality (meta:resourcekey, strongly-typed access, support for existing UserControls, et cetera), I provide a BuildProvider that allows default ASP.NET localization to live side-by-side with the DotNetNuke implementation. You can read more about it here.
For those that do not require such functionality, I recommend disabling .RESX and .resources compilation in your web.config, as it is by default.
Hope this helps!
Brandon