Is there a way to get details about this error? I turned off custom errors:

It looks like I am missing a skin file. How do I get it back? I used the one from the default install of DNN 6.2.4.
You could look at your DotNetNuke log4net config settings and check where you are logging errors. The default location is {DotNetNuke_Root_Folder}/Portals/_default/Logs/.
The log level by default is set on error so you can change that to "Debug" if you want by changing the element in the following config file {DotNetNuke_Root_Folder}/DotNetNuke.log4net.config
You can copy the skin's folder in your DotNetNuke installation zip file to your installed DotNetNuke website. Path to skins (DotNetNuke_Community_07.00.05_Install.zip\Portals_default\Skins)
Hope this helps.
I had this error also; on a local instance. After installing a DNN Template via the .visx file, IIS had set up a virtual directory for the desktopmodules folder. Removing the virtual directory cleared this right up in my case. Took a long time to troubleshoot, so hopefully this helps someone avoid the wild goose chase.