Notice (8) : session_start(): ps_files_cleanup_dir: opendir(/var/lib/php/sessions) failed: Permission denied (13) [in /home/exas-newdev/htdocs/newdev.exas.gr/vendor/cakephp/cakephp/src/Http/Session.php, line 362]
Warning (2) : session_start() [<a href='https://secure.php.net/function.session-start'>function.session-start</a>]: Session cache limiter cannot be sent after headers have already been sent (output started at /home/exas-newdev/htdocs/newdev.exas.gr/vendor/cakephp/cakephp/src/Error/Renderer/HtmlErrorRenderer.php:37) [in /home/exas-newdev/htdocs/newdev.exas.gr/vendor/cakephp/cakephp/src/Http/Session.php, line 362]
Warning (512) : Unable to emit headers. Headers sent in file=/home/exas-newdev/htdocs/newdev.exas.gr/vendor/cakephp/cakephp/src/Error/Renderer/HtmlErrorRenderer.php line=37 [in /home/exas-newdev/htdocs/newdev.exas.gr/vendor/cakephp/cakephp/src/Http/ResponseEmitter.php, line 71]
Warning (2) : Cannot modify header information - headers already sent by (output started at /home/exas-newdev/htdocs/newdev.exas.gr/vendor/cakephp/cakephp/src/Error/Renderer/HtmlErrorRenderer.php:37) [in /home/exas-newdev/htdocs/newdev.exas.gr/vendor/cakephp/cakephp/src/Http/ResponseEmitter.php, line 164]
Warning (2) : Cannot modify header information - headers already sent by (output started at /home/exas-newdev/htdocs/newdev.exas.gr/vendor/cakephp/cakephp/src/Error/Renderer/HtmlErrorRenderer.php:37) [in /home/exas-newdev/htdocs/newdev.exas.gr/vendor/cakephp/cakephp/src/Http/ResponseEmitter.php, line 197]
Warning (2) : Cannot modify header information - headers already sent by (output started at /home/exas-newdev/htdocs/newdev.exas.gr/vendor/cakephp/cakephp/src/Error/Renderer/HtmlErrorRenderer.php:37) [in /home/exas-newdev/htdocs/newdev.exas.gr/vendor/cakephp/cakephp/src/Http/ResponseEmitter.php, line 197]
Error: Could not start the session

Could not start the session 📋

RuntimeException
CORE/src/Http/Session.php at line 363 (edit)
            return false;
        }
        if (!session_start()) {
            throw new RuntimeException('Could not start the session');
        }
        $this->_started true;

If you want to customize this error message, create templates/Error/error500.php