To help you troubleshoot - does a very basic installation (with PHP’s built-in web server and sqlite) work on that server? I spent some time with a new Debian 10 virtual machine, and got Kanboard working with the built-in server/sqlite. I didn’t edit the Kanboard config files at all.
I installed PHP with the following command:
apt install php-sqlite3 php-gd php-mbstring php-json php-xml php-curl libapache2-mod-php <-- NOTE: I updated this to include libapache2-mod-php
I downloaded and unzipped kanboard-1.2.13, and in the kanboard folder I ran:
php -S localhost:8000
I was able to log in as admin/admin. If a basic installation like mine works for you, then perhaps there’s an error in the config.php (or MySQL configuration). If you’re using Apache, there could be an error in the Apache config too.