SOLUTION:
Add the following code to the file /etc/apache2/apache2.conf and then restart apache. (from the console or from hestia)
If you use the Nginx proxy it also works.
<Directory />
AllowOverride ALL
</Directory>
Or also:
<Directory />
AllowOverride none
</Directory>
If you use Webmail with Snappymail you can solve the problem by running the following command in the console to change the permissions:
chown -R hestiamail:www-data /etc/snappymail/data