Wordpress or Hosting Provider create invalid links

Question

I tried to upload my wordpress installation to a webhoster and face the problem of invalid links. Not every link seems to be affected, but many of them.

If i try to reach "/wp-admin" Backend to install a plugin, the plugin site loads correctly, but as soon as i try to upload a "plugin.zip" File, i get the following error:

404 Not Found

The requested URL /wp-admin/www.mydomain.com/wp-admin/update.php was not found on this server.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

Well one can see easily, that the url must be invalid - but how come?

In the table "wp_options" i updated the columns: "siteurl" and "home" to www.mydomain.com.

Thank you for your help!