The Acme bundle is by default only available in development and test environments. As long as you working on your localhost, it's not a problem. You are probably working in the development environment and in your app_kernel.php Acme\DemoBundle is set to initialize only when the environment is dev or test. So it's working just fine.
On your server you're not accessing your site through the localhost. that means the development environment is not available. Your working on a production environment. Like I explained above, the production environment has no Acme\DemoBundle initialized, so it can't find your user entity.
There are 2 possibilities:
go to your app-kernel.php:
find the line "$bundles[] = new Acme\DemoBundle\AcmeDemoBundle();" and delete it. Then add the following line ("new Acme\DemoBundle\AcmeDemoBundle(),") to the first array of bundles.
create a new bundle and add it to your app.kernel.php and put your User class in that bundle.
If you're just creating a website for personal use, the first option may be the quickest and best option, but if you want your bundles to be named nice, you should use the last.
good luck