This issue has been probably caused by the fact that your domain does not resolve to the correct server.
In order to solve this issue you need to log in your cPanel and go to the File Manager. There navigate to the configuration.php file in the public_html directory and edit it. You have to make the following change:
from
$mosConfig_live_site = 'http://mydomain.com';
to
$mosConfig_live_site = 'http://servername.com/~user';
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