the answer of the Dreamhost Support:
"Have you tried adding the following to an .htaccess file in your web
directory:
AddType text/cache-manifest appcache manifest
Also this article might be worth reviewing:
html5rocks.com/en/tutorials/appcache/beginner/%22
I created a .htaccess file with following content:
AddType text/cache-manifest appcache manifest
But the error persists. Is should be it?