problem with .htaccess file
That usually means something in your .htaccess is causing Apache to fail. Could for example be a missing module or a configuration that denies access to something.

Try to look at the error log of the Apache server, if you have access to it. If not (even many shared hosting sites allow access to those files for your virtual host), you could ask your hosting provider support.

It is only possible to provide a more detailed solution once you post the actual error from the error log.