I encountered the following error when I uploaded an existing site to a new server.
The server encountered an internal error or misconfiguration and was unable to complete your request.
The problem was the .htaccess file. I deleted the .htaccess and the web server worked again.
Subsequently, I recreated the file directly on the server using vim and the server still worked, I think therefore the problem was likely to be an issue with DOS carriage returns / line feeds versus what the Linux server expected.
Several tools exist on the web to help create .htaccess files online, but generally any text editor will work. Do be sure to test your site thoroughly after making an adjustments to the .htaccess file.