Problem setting up a codeigniter-project on webserver

Hi,

I have a codeigniter-project that I've been developing via my own computer with MAMP-server.
Now I am trying to move this project to a webserver under a subfolder.
When I upload the files and try to open the first screen, I get a 404 error.

Strange thing is that if I upload a clean codeigniter installation under another subfolder to the same server the project opens correctly with a welcome screen. So the server itself should be ok.

I have tried to define in config.php this:

$config['base_url'] = 'http://www.myserver.com/myproject';
$config['index_page'] = 'index.php';

and also this:
$config['base_url'] = '';
$config['index_page'] = 'index.php';

in routes.php I have this:
$route['default_controller'] = 'mainsite';

And I have that mainsite-controller in controllers-folder

If i change the default-controller:
$route['default_controller'] = 'welcome';

..I get an error: "Unable to locate the model you have specified: User_model"
I have added that User_model to autoload.php.

Any ideas?