Thanks for the quick answer.
I am sorry to spam this forum with all my commands but maybe it helps someone else and it hopefully helps you guy troubleshooting the issues.
So I go back to my snapshot1 and install the missing php extension.
sudo apt install php-mysql
cd /usr/share/nginx/invoiceninja/
sudo rm invoiceninja.zip
sudo -u www-data wget https://github.com/invoiceninja/invoiceninja/releases/download/v5.5.14/invoiceninja.zip
sudo shutdown now
create snapshot2
cd /usr/share/nginx/invoiceninja/
sudo -u www-data unzip invoiceninja.zip
sudo rm invoiceninja.zip
sudo rm -r bootstrap/cache/*
sudo -u www-data php artisan optimize
Everything seems to work perfectly! Thanks a lot for your help! Awesome support as always @david and @hillel! Seriously guys, I never saw any other software that had such great support without a service contract. You put a lot of effort into this forum and I hope it also pays out for you! 