I tried with 777 and it doesn't work. The .htaccess was generated on the previous server. I tried to access the admin with that version of .htaccess and no luck. After that I renamed it but the error still occurs.

Clear your browser cache and your cookies. Have you already checked your database rows: 'PS SHOPDOMAIN' and 'PS SHOPDOMAINSSL' inside the 'ps configuration table'?

If that doesn't work you can manually clear smarty cache. To remove the cache you have to remove all files from 'tools/smarty/compile' and 'tools/smarty/cache'. Don't forgot to make a back-up of your current files to roll back.

I tried with different browsers but no luck. The database is in good shape and the shop URLs are correct. The website was working smoothly before the migration.

Have you manually removed the cache like i said before? Because i didn't notice something about that in your previous reaction. If that still didn't worked you can try to disable the IP check.

To disable the the IP check: open classes/Cookie.php.

// Remove or command out

AND (!isset($this->_content['remote_addr']) OR $this->_content['remote_addr'] == ip2long(Tools::getRemoteAddr()) OR !Configuration::get('PS_COOKIE_CHECKIP'))

Removed that line from Cookie.php -> same error. I deleted all files from cache/smarty/cache and cache/smarty/compile (except the index.php) and still nothing

Maybe a late question, but did you enabled htaccess in the apache config file?

<Directory /path/to/public_html>
        Options Indexes FollowSymLinks
        AllowOverride All
        Require all granted
</Directory>

Did you also run enable mod rewrite?

sudo a2enmod rewrite
sudo service apache2 restart

It might not be needed. I retried the permissions that you suggested. 644 to all files and 755 to folders.

find /var/www/testapp/public_html/cache -type d -exec chmod 755 {} \;
find /var/www/testapp/public_html/cache -type f -exec chmod 644 {} \;

It seems that Smarty is checking that the files and/or folders have that exact permission. I'm really curious how come 777 didn't worked.

The admin is now working.
Thanks Matthew

Your're welcome, probably it is an security feature.

find /var/www/testapp/public_html/cache -type d -exec chmod 755 {} \;
find /var/www/testapp/public_html/cache -type f -exec chmod 644 {} \;