ERROR: 0 class 'Joomla\CMS\Factory' not found

[xbregtje]

Hello!
For a friend I tried to update Joomla to Joomla 3.6.1.
That failed, and I tried to restore the site with Akeeba backup kickstart.
Sadly, I get the same error as after updating Joomla:

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ERROR: 0 class 'Joomla\CMS\Factory' not found'.

After enabling the '$error_reporting' to maximum I get the following error:

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Deprecated: Array and string offset access syntax with curly braces is deprecated in /var/www/vhosts/fysiotherapie-fysiek.nl/webspace/httpdocs/fysiotherapie-fysiek.nl/libraries/joomla/database/driver.php on line 1947 Warning: ini_set(): Headers already sent. You cannot change the session module's ini settings at this time in /var/www/vhosts/fysiotherapie-fysiek.nl/webspace/httpdocs/fysiotherapie-fysiek.nl/libraries/joomla/session/handler/joomla.php on line 45 Warning: session_set_cookie_params(): Cannot change session cookie parameters when headers already sent in /var/www/vhosts/fysiotherapie-fysiek.nl/webspace/httpdocs/fysiotherapie-fysiek.nl/libraries/joomla/session/handler/joomla.php on line 139 Warning: session_set_save_handler(): Cannot change save handler when headers already sent in /var/www/vhosts/fysiotherapie-fysiek.nl/webspace/httpdocs/fysiotherapie-fysiek.nl/libraries/joomla/session/storage.php on line 90 Warning: session_name(): Cannot change session name when headers already sent in /var/www/vhosts/fysiotherapie-fysiek.nl/webspace/httpdocs/fysiotherapie-fysiek.nl/libraries/joomla/session/handler/native.php on line 127 Warning: ini_set(): Headers already sent. You cannot change the session module's ini settings at this time in /var/www/vhosts/fysiotherapie-fysiek.nl/webspace/httpdocs/fysiotherapie-fysiek.nl/libraries/joomla/session/session.php on line 910 Warning: session_cache_limiter(): Cannot change cache limiter when headers already sent in /var/www/vhosts/fysiotherapie-fysiek.nl/webspace/httpdocs/fysiotherapie-fysiek.nl/libraries/joomla/session/handler/native.php on line 233 Error displaying the error page: Application Instantiation Error: Failed to start the session because headers have already been sent by "/var/www/vhosts/fysiotherapie-fysiek.nl/webspace/httpdocs/fysiotherapie-fysiek.nl/libraries/joomla/database/driver.php" at line 1947.

I tried to fix the braces by hand, but ended up being a very bad person / hacker according to Joomla.

Can anybody help me?

Info:
- Akeeba Backup version: 6.4.0
- PHP version: 7.4.33
- Joomla! version: 3.5.1